POLE  AND  TOWER  LINES 


McGraw-Hill  BookCompany 


Electrical  World         The  Engineering  and  Mining  Journal 
Engineering  Record  Engineering  News 

Railway  Age  G azette  American  Machinist 

Signal  Hngin<?Gr  ^American Engineer 

Electric  Railway  Journal  Coal  Age 

Metallurgical  and  Chemical  Engineering  Power 


POLE  AND 
TOWER   LINES 

FOR 

ELECTRIC   POWER 
TRANSMISSION 


BY 
R.  D.  COOMBS,  C.  E. 

CONSULTING    ENGINEER 

MEMBER   AMERICAN    SOCIETY    OF     CIVIL  ENGINEERS,   AMERICAN 
RAILWAY  ENGINEERING  ASSOCIATION,  AMERICAN  ELEC- 
TRIC   RAILWAY    ASSOCIATION,    AND    NATIONAL 
ELECTRIC     LIGHT   ^ASSOCIATION 


FIRST  EDITION 


McGRAW-HLLL  BOOK  COMPANY,  INC. 
239  WEST  39TH  STREET,  NEW  YORK 

6  BOUVERIE  STREET,  LONDON,  E.  C. 

1916 


COPYRIGHT,  1916,  BY  THE 
MCGRAW-HILL  BOOK  COMPANY,  INC. 


THE    MAPI>E     PRESS     YORK    PA 


PREFACE 

The  rapid  growth  of  the  electric  light  and  power  industry 
within  the  last  decade  has  caused  an  enormous  increase  in  both 
the  number  and  length  of  transmission  and  distribution  lines. 
From  a  comparatively  unimportant  detail — of  little  interest 
even  to  the  owner— the  power  lines  have  developed  into  quite 
pretentious  systems  whose  relation  to  other  industries  and  to 
the  public  must  be  taken  into  consideration.  The  occasional 
transmission  systems  designed  for  11,000  volts  and  consisting  of 
wood  pole  lines  with  spans  of  120  ft.  have  developed  into  150,000- 
volt  lines  on  steel  towers  with  spans  from  500  to  800  ft.  long, 
while  the  light  wooden  poles  supporting  a  few  street  lighting 
circuits  have  been  superseded  in  some  cases  by  very  heavy  trunk 
lines  of  many  cables.  As  is  to  be  expected  in  a  growing  industry, 
no  set  of  standards  has  been  universally  adopted.  Moreover, 
it  is  impossible  to  apply  any  one  specification  or  standard  of 
construction  universally,  unless  such  a  standard  has  some 
elasticity  and  is  interpreted  and  enforced  intelligently.  In  any 
attempt  at  standardization,  either  for  one  operating  company 
or  in  a  national  specification,  it  is  necessary  to  consider  carefully 
the  cost  and  the  operating  problems  involved  in  the  adoption 
of  any  general  mechanical  requirements.  An  apparently  harm- 
less requirement  may  become  a  very  serious  matter  if  applied  to 
all  future  construction. 

The  number,  size  and  voltage  of  wires  and  the  type  of  insula- 
tion to  be  used  belong  to  the  field  of  electrical  engineering, 
while  the  subsequent  determination  of  the  best  method  of  carry- 
ing the  conductors  across  country  is  purely  a  question  of  civil 
engineering.  The  terms  "electrical"  and  "civil"  are  used  in 
their  narrow  sense,  but  the  division  is  important  as  denoting  the 
proper  apportioning  or  departmentizing  of  the  work.  In  this 
age  of  specialists  there  is  no  more  reason  for  electrical  engineers 
to  determine  structural  details,  than  there  is  for  structural 
engineers  to  decide  on  the  proper  type  of  insulator.  If  laymen 
or  workers  in  other  branches  of  the  engineering  profession  have 
hesitated  to  assume  authority  in  electrical  matters,  the  same 
cannot  be  said  of  the  average  electrical  expert  in  the  field  of  pure 

v 

331010 


vi  PREFACE 

construction.  This,  no  doubt,  is  due  to  the  fact  that  those 
engaged  in  coordinate  branches  of  the  profession  have  done  little 
or  nothing  to  solve  the  problems  properly  assignable  to  construc- 
tion engineers. 

That  this  is  no  imaginary  charge  is  substantiated  by  the 
history  of  transmission  line  construction.  Until  recent  years 
the  rule-of-thumb  practices  of  telephone  companies — worthy 
results  of  the  test  of  time  as  many  of  them  are — were  followed 
blindly  by  those  in  charge  of  electrical  transmission.  As  a 
result  there  are  many  improperly  constructed  lines  and  erroneous 
ideas  prevail  regarding  the  facts  and  principles  involved  in  their 
design.  This  condition  should  no  longer  be  allowed  to  exist. 

It  is  not  the  purpose  of  the  writer  to  deal  with  purely  electrical 
problems,  such  as  the  relation  of  voltage  and  size  of  the  wires  to 
the  electrical  characteristics  of  a  line,  or  with  such  very  specialized 
matters  as  the  design  of  insulators.  The  problem  is  rather  to 
develop  a  clearer  preception  of  the  application  of  the  laws  of 
mechanics  to  the  case  in  hand. 

The  writer  wishes  to  acknowledge,  with  thanks,  the  assistance 
rendered  by  Mr.  W.  L.  Cadwallader  in  preparing  the  tables  and 
computations,  and  by  those  in  charge  of  various  properties  in 
furnishing  illustrations  and  data  relating  thereto. 

R.  D.  COOMBS. 

NEW  YORK,  N.  Y. 
1916. 


CONTENTS 

PAGE 

PREFACE    v 

CHAPTER  I 

TYPES  OF  CONSTRUCTION 1 

Carrying  Capacity  of  Supports — Life  of  Materials — Clearances — 
Tree  Trimming — Right-of-way — Factor  of  Safety — Spans — Sup- 
ports— Location  Plan. 

CHAPTER  II 

LOADING 29 

Sleet— Wind^-Broken  Wires. 

CHAPTER  III 

WIRES  AND  CABLES 43 

Copper — Copper  Covered — Aluminum — Steel — Telephone  Wire — 
Catenary. 

CHAPTER  IV 

DESIGN 60 

Factors  of  Safety,  Etc. — Transverse  Loads — Corner  Loads — 
Broken-wire  Loads — Column  Formulas — Strength  of  Wooden  Poles. 

CHAPTER  V 

WOODEN  POLES 76 

Decay  and  Defects — Seasoning — Preservatives — Pressure  Treat- 
ment— Open-tank  Process — Brush  Treatment — Framing  of  Poles 
— Design  of  Wood  Poles — A-frames  and  H-frames. 

CHAPTER  VI 

STEEL  POLES  AND  TOWERS 103 

Rivets  and  Bolts — Lacing — Angle  Lacing — Tower  Connections — 
Latticed  Poles — Curb-line  Poles — Triangular  Poles — Wide-base 
Towers — Flexible  Frames. 

CHAPTER  VII 

SPECIAL  STRUCTURES 141 

Transposition — Outdoor  Sub-stations — Switching  Stations,  Etc. — 
High  Towers — Aerial  Cable. 

vii 


viii  CONTENTS 

CHAPTER  VIII 

PAGE 

CONCRETE  POLES .    .    .    .    152 

General  Considerations — Tests — Sections — Strengths. 

CHAPTER  IX 

FOUNDATIONS 165 

General — Wood  Pole  Settings — Bog  Shoes — Barrel  Foundations — 
Concrete  Settings — Tower  Foundations — "A "-frame  Foundations 
— Rock  Foundations — Concrete — Cement — Proportions — Aggre- 
gates— Water- — Mixing  and  Placing — Forms — Workmanship — 
Reinforcement — Waterproofing,  Salt  Water,  Alkali.  Etc. 

CHAPTER  X 

PROTECTIVE  COATINGS ;    .......   180 

Paint  and  Painting — Galvanizing — Standard  Specifications  for 
Spelter — Standard  Specifications  for  Galvanizing. 

CHAPTER  XI 

LINE  MATERIAL 189 

Tie  Wires — Loops — Splices — Pin  Insulators — Pins — Crossarms — 
Crossarm  Braces — Lag  Screws  or  Lag  Bolts — Guys  and  Guying — 
Guy  Anchors. 

CHAPTER  XII 

ERECTION  AND  COSTS 217 

Erection — Costs. 

CHAPTER  XIII 

PROTECTION      232 

Ground  Wires — Neighboring  Lines — Cradles — Clamping  Devices 

CHAPTER  XIV 

SPECIFICATIONS 248 

Joint  Report  Specifications  for  Crossings — General  Specifications 
for  Lines — Galvanizing  (see  Chapter  X). 

INDEX  .  .   269 


POLE  AND  TOWER  LINES 

CHAPTER  I 
GENERAL  CONSTRUCTION 

The  choice  of  a  type  of  construction  for  transmission  lines,  on 
private  rights-of-way,  may  be  said  to  depend  on  the  cost  of  con- 
struction versus  the  cost  of  maintenance  and  of  interruptions  to 
service.  This  is  also  true,  though  to  a  less  extent,  of  certain 
classes  of  railroad  power  lines.  Where  the  lines  of  a  transmission 
company  are  on  public  property,  or  cross  other  lines,  or  when 
high- voltage  lines  are  used  by  railroads  for  electric-traction,  the 
cost  of  failures  of  construction  will  be  found  to  exceed  the  addi- 
tional cost  of  good  construction. 

A  decision  as  to  the  exact  type  of  line,  character  of  supports, 
and  total  wire  capacity  which  will  provide  the  highest  ultimate 
economy,  as  well  as  excellence  of  service  is  perhaps  impossible. 
A  designer  can  only  use  good  judgment  in  estimating  future 
developments,  and  engineering  skill  on  the  immediate  construc- 
tion. The  number  of  years  that  any  particular  line,  perhaps  any 
or  all  lines,  will  need  to  remain  in  the  original  location  cannot  be 
definitely  foretold.  If  the  line  remains  in  place  for  a  long  period 
of  years,  will  it  have  any  considerable  scrap  value  at  the  end  of 
that  period?  If  removed,  or  considerably  altered,  in  a  shorter 
period,  the  value  remaining  in  the  material  is  scarcely  less  diffi- 
cult to  determine.  Apart  from  the  economic  needs  of  the  owner 
10  or  20  years  hence,  there  is  the  question  of  the  effect  upon 
the  existence  of  the  line,  of  federal  or  municipal  regulation  at 
some  earlier  date. 

In  addition  to  these  considerations,  there  must  also  be  taken 
into  account  the  probable  future  wire-carrying  capacity  and 
voltage,  the  life  of  the  materials,  and  finally  the  relative  ultimate 
economy  of  one  or  another  type  of  construction  as  judged  by  the 
above  conditions. 

In  order  to  obtain  a  better  understanding  of  the  various  fac- 
tors which  affect,  from  a  construction  standpoint,  a  decision  as 
to  the  type  of  line,  the  following  details  may  be  noted : 

1 


POLE  AND  TOWER  LINES 


Carrying  Capacity  of  Supports. — Experience  with  the  lower 
voltages,  at  least,  has  shown  that  the  capacity  of  the  supports  to 

carry  additional  wires  has  usu- 
ally been  underestimated.  The 
extremely  heavy  distribution 
lines  often  seen  on  city  streets 
are  not,  however,  an  example  of 
a  proper  ultimate  wire  capacity, 
but  rather  of  an  unwise  over- 
loading. Exact  maximum  limits 
cannot  be  set,  but  the  occasional 
failure  of  such  overloaded  lines 


FIG.  1. — Overloaded  wooden  poles. 


FIG.  2. — One-circuit  wooden  poles, 
33,000  volts. 


causes  an  expenditure  which  should  be  a  weighty  argument  for 
the  alternative  of  more  and  lighter  lines.  It  is  true  that  two 
light  lines  will  cost  more  than  one  heavy  line  in  so  far  as  the  actual 


GENERAL  CONSTRUCTION  3 

construction  cost  is  concerned,  but  the  increased  maintenance  of 
the  latter  and  its  greater  liability  to  extensive  or  prolonged  inter- 
ruption to  service,  if  difficult  to  foreteh1  in  terms  of  money,  are 
not  less  costly  on  that  account.  Many  otherwise  competent 
engineers  seem  to  ignore  the  very  real  cost  in  time,  money,  and 
reputation  " saved"  by  a  reduction  in  the  initial  construction 
account. 

The  ordinary  wood-pole  distribution  lines  are  subject  to  more 
alteration  than  transmission  lines,  and  are  affected  by  conditions 
not  generally  applicable  to  transmission  lines.  In  transmission- 
line  construction,  the  question  is  whether  to  provide  for  one  or 
two,  two  or  four,  or  four  or  more  circuits  upon  a  single  line  of 
supports,  and  in  the  higher  voltages  whether  to  carry  one  or  two 
circuits. 

The  financial  ability  or  disability  of  the  owner  may  settle  the 
question  of  the  number  of  circuits  without  reference  to  any  further 
considerations  whatever.  The  best  may  be  the  cheapest,  but  it 
may  also  be  the  impossible.  Under  such  circumstances,  the 
engineer  can  build  only  as  well  as  is  practicable  with  the  funds 
available.  Lines  built  under  such  restrictions  are  usually  in  less 
populous  territory,  and  the  penalties  of  accident  are  less  severe 
than  in  the  neighborhood  of  cities.  The  natural  and  entirely 
proper  practice  will  be  to  provide  relatively  low  supports,  long 
spans,  light  structures,  and  very  little  future  capacity.  It  may 
be  noted,  however,  that  proper  side  clearances,  spacing,  insula- 
tion, and  guying,  add  but  little  to  the  cost  compared  to  the  gain 
in  the  efficiency  of  the  line. 

In  what  may  be  called  the  better  grades  of  construction,  the 
number  of  circuits  upon  a  single  support  depends  very  largely 
upon  the  location  of  the  line  and  its  present  or  future  voltage. 
There  is  a  quite  important  difference  between  lines  upon  private 
rights-of-way  and  those  upon  highways  or  private  property.  It 
is  not  unreasonable  to  assume  that  an  operating  company  may 
utilize  any  combination  of  heavily  loaded  or  duplicated  lines 
it  chooses  upon  its  own  property,  though  that  property  will 
eventually  have  to  be  fenced  and  patrolled.  On  the  other  hand, 
the  occupation  of  highways,  etc.,  by  overloaded  or  perhaps  any 
lines,  may  become  subject  to  criticism.  Again  the  holding  of 
separated  parallel,  or  looped,  lines  has  real  advantages  from  the 
commercial  and  service  standpoint. 

Dividing  the  total  capacity  for  wires  between  two  or  more  pole 


4  POLE  AND  TOWER  LINES 

lines  cannot  be  justified  as  a  measure  of  reducing  the  cost.  Such 
duplicate  lines  will  always  cost  more  than  one  heavy  line,  but 
the  subdivision  may  be  justified  on  other  grounds.  In  case  one 
line  is  given  a  different  route,  the  maximum  security  from  simul- 
taneous interruption  to  service  is  obtained  and  additional  busi- 


FIG.  3. — Two-circuit  wooden  poles,  33,000  volts. 

ness  may  be  secured,  but  the  cost  will  be  more  than  that  of  two 
lines  on  one  right-of-way,  and  nearly  double  that  of  one  line  of 
the  total  capacity. 

The  number  of  circuits  that  may  be  carried  by  a  single  support 
depends  on  the  electrical  characteristics,  the  continuity  of  service 


GENERAL  CONSTRUCTION  5 

desired  and  to  some  extent  on  thfc  length  of  span  and  the  charac- 
ter of  the  supports.  Thus  the  same  weight  of  conductor  divided 
between  two  circuits  affords  considerable  protection  against 
interruptions  to  service,  with  little  added  cost  except  for  higher 
poles,  larger  or  more  numerous  arms,  more  insulators,  etc.  A 
single  circuit,  however,  can  be  arranged  with  more  space  between 
conductors  and  may  be  used  more  economically  in  long  spans. 

Life  of  Materials. — In  general  it  may  be  assumed  that  a  line 
should  have  the  longest  life  consistent  with  a  reasonable  first 
cost,  and  that  after  selecting  a  type  of  construction  and  factors  of 


FIG.  4. — Two  one-circuit  lines,  55,000  volts. 

safety  proper  for  the  location  under  consideration,  the  matter  of 
restrictive  regulation  must  be  left  to  the  future. 

In  connection  with  the  probable  life  of  materials  the  statement 
is  frequently  made  that  the  life  of  steel  is  indefinite.  This  is 
literally  correct  though  not  exactly  as  intended.  The  life  of 
properly  protected  steel  is  indefinitely  long,  while  that  of  steel 
exposed  to  the  weather  and  unprotected  is  indefinitely  short. 
Many  galvanized  wind  mills  which  are  20  years  old  are  still  in 
good  condition,  and  some  painted  steel  structures  are  much  older. 
On  the  other  hand,  there  are  both  galvanized  and  painted  struc- 
tures whose  probable  life  will  be  less  than  20  years.  The  probable 


6 


POLE  AND  TOWER  LINES 


average  life  of  unprotected  timber  poles  is  about  10  years,  while 
that  of  poles  which  have  been  properly  treated  with  preserva- 
tives is  from  16  to  20  years,  depending  on  the  treatment.  As 
reinforced-concrete  poles  are  a  more  recent  development  there 
has  not  been  the  same  opportunity  to  obtain  fair  averages,  but 


FIG.  5. — Two  one-circuit  steel  towers,  60,000  volts. 

from  observing  installations  10  years  old  it  appears  that  their 
average  life  is  comparable  with  that  of  steel. 

Clearances. — Until  recent  years  the  only  general  clearance 
specified  by  engineers,  or  required  by  law,  was  that  power  wires 
should  be  maintained  at  a  height  of  25  ft.  above  highways  or 


I 
i 


GENERAL  CONSTRUCTION 


above  the  top  of  rail  over  railroad  tracks.  In  some  cases  a  clear 
height  of  22  ft.  was  permitted.  Such  clearances,  like  most  of  the 
early  requirements  in  transmission  construction,  were  founded 
on  and  often  a  direct  copy  of  existing  telephone  and  telegraph 
practice. 

The  overhead  clearance  necessary  to  permit  the  trolley  pole 
on  a  large  modern  car  to  swing  into  its  upright  position,  in  case  it 


FIG.  6.— Two-circuit  steel  towers,  110,000  volts. 

accidently  leaves  the  trolley  wire,  would  perhaps  indicate  the 
minimum  limit.  While  overhead  systems  for  alternating-current 
operation  of  railroads  or  interurban  lines  are,  as  yet,  but  few  in 
number — the  largest  and  best  known  installation  being  that  on 
the  New  York  Division  of  the  New  York,  New  Haven  &  Hartford 
Railfoad — such  construction  should  receive  consideration  in 
establishing  a  standard  overhead  clearance  above  railroad 


8  POLE  AND  TOWER  LINES 

tracks.  In  such  cases  the  railroad  company  would  probably 
desire  to  have  their  transmission  circuits,  if  on  a  separate  pole 
line,  carried  above  all  crossing  lines,  except  possibly  those  of 
very  high  voltage  systems.  However,  railroad  trolley-contact 
wires  cannot  be  elevated  above  other  crossing  wires.  The  mini- 
mum height  of  such  overhead  contact  wires  is  approximately 
23  ft.  at  the  center  of  a  span  and  28  ft.  at  the  supports. 

Linemen  working  on  foreign  wires,  whether  on  joint-pole  lines 
or  not,  should  be  protected  from  contact  with  power  wires  by 
providing  a  reasonable  space  between  the  two  lines  or  sets  of 
wires.  In  this  connection  mention  may  be  made  of  one  reason 
for  requiring  telephone  and  telegraph  wires  to  be  placed  below 
power  wires  either  at  crossings  or  when  located  on  joint  poles, 
i.e.,  to  prevent  the  harmless  wires  dropping  into  contact  with  the 
power  wires,  either  during  the  process  of  stringing  wire  or  due 
to  the  more  frequent  mechanical  failure  of  the  smaller  telephone 
wires. 

It  should  be  remembered  that  the  stresses  on  poles  increase 
directly  with  their  height  and,  in  fact,  more  rapidly  than  in  a 
direct  ratio  when  there  is  no  adjoining  protection  from  the  wind. 
Omitting  considerations  of  accidental  contact  and  malicious  in- 
jury, the  lower  line  is  usually  the  safer  line.  Assuming  that  the 
power  or  lighting  circuit  is,  as  it  should  be,  the  higher  circuit,  or 
in  the  case  of  several  lines  of  different  voltages  that  they  are 
arranged  in  order  with  the  highest  voltage  circuit  uppermost, 
there  should  remain  below  the  power  wires  a  zone  for  harmless 
wires.  This  arrangement  is  especially  necessary  over  highways 
or  railroads  where  such  inferior  lines  exist  or  may  be  reasonably 
expected  in  the  near  future. 

The  proper  separation  of  conductors  to  prevent  their  swinging 
into  contact  has  never  been  definitely  determined.  It  has  been 
argued  that  long  spans  swing  synchronously,  and  that  experience 
has  shown  that  they  may  be  safely  spaced  much  closer  together 
than  the  distance  required  to  provide  for  the  maximum  displace- 
ment. On  the  other  hand,  short  spans  with  relatively  greater 
separation  have  been  brought  into  contact  by  what  appears  to 
be  a  purely  electrical  cause. 

It  is  doubtful  whether  engineers  and  executives  realize  the 
extent  to  which  undesirable  construction  is  installed,  due  to 
inadequate  efforts  to  obtain  the  necessary  concessions  on  the  part 
of  outside  interests.  Isolated  trees,  of  perhaps  no  particular 


GENERAL  CONSTRUCTION 


9 


value,  have  compelled  the  use  of  high  poles  or  the  unnecessary 
grading  up  of  pole  lines.  Without  wishing  to  appear  an  advocate 
of  some  of  the  common  methods  of  "tree  trimming,"  the  writer 
believes  that  one  large  scraggly  tree,  more  or  less  decayed,  re- 
maining along  a  curb  line  where  the  other  trees  are  of  recent 


FIG.  7. — Steel  pole  line,  11,000  volts,  with  space  for  lower  voltages. 

regular  growth,  should  not  be  allowed  to  interfere  with  the  proper 
location  of  all  the  wires  in  that  street. 

Everywhere  throughout  the  country,  there  are  towns  in  which 
the  telephone  and  electric  service  lines  occupy  all  sorts  of  zones, 
gradually  getting  higher  and  higher,  until  the  latest  line  is  driven 
to  pole  heights  extremely  difficult  to  obtain.  It  will  also  be 
found  that  many  telephone  lines  have  overbuilt  the  present 


10 


POLE  AND  TOWER  LINES 


power  wires  and  occupy  the  zone  which  should  be  used  by  future 
power  wires  of  higher  voltage.  Telephone  or  telegraph  wires 
should  be  placed  underneath  power  wires,  as  it  is  impracticable 
to  give  the  former  a  sufficient  factor  of  safety  to  prevent  mechan- 
ical failure. 

There  is  no  established  relationship  between  the  span  and 
sag,  and  the  separation  between  conductors.  Two  general 
methods  have  been  advanced  for  determining  the  separation  be- 
tween conductors:  (1)  depending  on  sag  and  span,  and  (2)  on 
sag  only.  The  first  is  incorporated  in  the  Pennsylvania  Railroad 
wire-crossing  specifications  which  prescribe  a  separation  of  1  in. 


.5  5 


fi  4 


01     234     5     6     78     9    10  11  12  13  14  15  16  17  18  19  20 
Sag  in  Feet 

FIG.  7 A  — Conductor  separations. 

for  each  20  ft.  of  span  plus  1  in.  for  each  foot  of  sag  or  fraction 
thereof.  The  second  method,  proposed  by  the  writer  in  a  paper 
before  the  National  Electric  Light  Association  convention  in  1913, 
is  shown  in  Fig.  la. 

In  both  methods  mentioned,  the  minimum  separation  may  not 
be  less  than  that  given  for  shorter  span  lengths  in  paragraph  3 
of  the  specifications  on  page  259.  The  use  of  suspension  insula- 
tors involves  an  additional  separation  of  one  and  one  quarter 
times  the  length  of  the  string  of  suspension  insulators. 

Tree  Trimming. — In  order  to  provide  and  maintain  adequate 
separation  between  conductors  and  adjoining  timber,  a  rather 
indefinite  amount  of  tree  cutting  and  trimming  must  be  done. 
The  actual  amount  of  such  work  will  vary  between  wide  limits 


GENERAL  CONSTRUCTION  11 

for  different  lines  even  in  the  same  locality.  For  lines  located  on 
streets  the  problem  is  first  to  select  the  most  accessible  and  least- 
shaded  street,  and  second,  to  adjust  the  height  of  the  poles  and 
the  amount  and  character  of  trimming  so  as  to  obtain  the  maxi- 
mum protection  with  the  minimum  of  offense.  In  cross- 
country lines,  however,  particularly  those  on  private  right-of-way, 
the  problem  is  somewhat  different.  In  this  case,  there  is  usually 
some  freedom  of  movement  through  which  the  line  may  be  so 
located  as  to  avoid  too  close  proximity  while  retaining  the  benefit 
of  distant  shelter.  This  latter  feature  seems  to  have  been  gen- 
erally disregarded,  and  yet,  provided  sufficient  separation  is  main- 
tained to  prevent  falling  contacts,  the  presence  of  timber  land  to 
windward  is  in  the  writer's  opinion  a  considerable  asset  in  the 
strength  of  ordinary  lines. 

Cutting  down  trees,  while  generally  deplorable,  cannot  always 
be  avoided,  so  its  justification  must  necessarily  depend  on  the 
quality  of  the  interfering  tree  and  the  importance  and  position 
of  the  line.  Some  trees  will  so  outrank  the  ordinary  power  line, 
both  in  their  real  and  in  their  popular  value,  that  a  change  in 
route  may  have  to  be  considered.  Other  trees  are  past  their 
prime  and  have  merely  a  sentimental  value  to  a  limited  number 
of  persons.  In  some  cases  permission  for  indiscriminate  cutting 
will  be  freely  given.  Either  in  cutting  or  in  trimming,  a  broad- 
minded  liberal  policy  on  the  part  of  the  power  company,  coupled 
with  considerable  tact,  will  ultimately  justify  itself. 

Unpruned  trees  with  long  scraggly  limbs,  instead  of  being 
injured,  will  generally  be  improved  by  proper  trimming.  Dead 
or  dying  branches  are  of  no  benefit  to  trees,  whereas  they  are  a 
serious  menace  to  the  power  company;  therefore  they  should  be 
removed  in  the  immediate  neighborhood  of  the  line. 

The  methods  in  vogue  in  trimming  trees  are  greatly  in  need  of 
improvement.  Aside  from  the  serious  loss  in  popularity  and 
prestige,  it  is  nothing  short  of  criminal  waste  to  unnecessarily 
injure  grown  timber.  This  country  once  possessed  enormous 
forests,  yet  the  present  timber  lands  are  only  a  pitiful  remnant 
and  cultivation  is  almost  unknown.  In  pole-timber  land  it  is  no 
exaggeration  to  claim  that  every  tree  unnecessarily  cut  down  or 
killed  adds  its  mite  to  the  future  maintenance  expenditures  of 
the  local  power  company. 

By  exercising  some  care  it  is  possible  to  trim  so  that  killed 
trees  or  non-permanent  clearances  should  be  rare.  The  season 


12  POLE  AND  TOWER  LINES 

of  the  year  in  which  trimming  is  done  has  a  marked  influence  on 
the  successful  healing  of  the  cuts.  In  general  it  is  best  to  trim 
any  tree  during  the  dormant  season. 

In  removing  large  limbs  they  should  be  first  undercut  to  pre- 
vent slivering  and  then  sawed  through  close  to  the  trunk.  The 
stump  should  then  be  cut  off  flush  with  the  trunk,  leaving  a 
smooth  surface,  which  may  be  painted  when  it  has  dried.  If  this 
is  not  done,  the  stump  will  decay  and  the  rot  will  spread  to  the 
trunk.  Small  limbs  may  be  cut  immediately  beyond  any  forks, 
or  close  to  the  trunk;  upright  limbs  should  be  cut  on  a  slant  and 
the  surface  should  be  finished  smooth  and  then  painted. 

When  an  entire  tree  is  to  be  cut  down,  the  stump  should  be 
short  and  be  given  a  smooth  ridge  or  roof  similar  to  that  on  a  pole 
top.  Second-growth  trees  will  then  usually  sprout  from  the 
stumps  and  be  available  for  lumber  in  the  future. 

In  protecting  or  patching  rotten  cavities,  the  dead  wood  should 
be  cut  away  to  form  a  pocket  with  undercut  edges  as  in  dental 
work,  and  the  cavity  should  then  be  painted  and  left  as  an  open 
cavity  or  be  filled  with  cement  mortar  or  asphalt. 

On  private  right-of-way  all  trees,  brushwood,  sage  brush,  etc., 
should  be  cut  down  and  cleared  away.  The  amount  of  cutting 
on  neighboring  property  will  vary  greatly,  but  should  in  all  cases 
include  the  trimming  of  nearby  dead  branches  and  such  trees 
as  by  their  unusual  position  create  a  particular  hazard. 

The  attachment  of  guys,  without  tree  blocks  or  shields,  will 
frequently  kill  a  part  or  all  of  a  tree,  lessen  its  value  as  a  stub,  and 
also  render  it  a  menace  to  the  line. 

Trees  grow  by  the  addition  of  wood  fiber  to  the  outside  of 
existing  limbs,  and  the  new  growth  is  fed  by  a  descending  liquid 
in  a  thin  layer,  called  the  cambium,  immediately  inside  the  bark. 

If,  therefore,  the  continuity  of  the  cambium  layer  is  broken  the 
new  wood  is  starved  just  below  the  break.  If  the  cambium  of  all 
or  a  large  proportion  of  the  circumference  is  cut  through  or  is 
prevented  from  growing  with  the  tree,  the  entire  tree  starves. 

Nearly  all  trees  are  dormant  in  winter  and  may  then  be  cut 
without  excessive  bleeding  and  without  subjecting  the  wounds  to 
attack  by  insects. 

Right-of-way. — The  simplest  form  of  right-of-way  is  that  of 
pole  or  tower  rights,  whether  obtained  along  streets  by  franchise 
from  the  municipality  or  by  lease  or  purchase  from  private 
owners.  Although  this  form  is  by  far  the  most  common,  it  seems 


GENERAL  CONSTRUCTION  13 

probable  that  there  will  be  a  marked  increase  in  the  number  of 
private  rights-of-way  for  both  pole  and  tower  lines.  Private 
rights-of-way,  while  naturally  more  expensive  in  original  cost, 
permit  the  use  of  the  most  economieal  types  of  construction  and 
provide  insurance  against  restrictive  regulation  or  an  excessive 
cost  for  increased  facilities.  The  abnormal  expenditures  in- 
volved in  hurried  construction  and  the  excessive  payments  often 
required  to  complete  a  right-of-way — a  species  of  blackmail — are 
perhaps  not  fully  realized.  Such  expenses  would  be  greatly 
reduced  in  the  case  of  a  private  right-of-way,  particularly  for  sub- 
sequent lines  installed  thereon.  It  might  be  argued  that  ex- 
cessive payments  would  be  demanded  for  a  continuous  right-of- 
way,  as  is  usually  the  case  in  railroad  construction.  While  such 
unit  prices  are  unquestionably  excessive  for  the  land  as  such, 
they  may  not  be  excessive  for  an  electric  right-of-way — at  least 
this  has  been  the  case  with  railroads.  Heretofore,  private  rights- 
of-way  have  been  purchased  chiefly  where  land  was  very  cheap 
and  when  an  important  line  on  wide-base  towers  was  to  be  con- 
structed. It  is  probable,  however,  that  equally  effective  reasons 
may  be  advanced  for  private  right-of-way  in  more  settled  com- 
munities, on  which  to  build  a  series  of  pole  lines. 

There  seems  to  be  no  general  standard  or  set  of  rules  by  which 
the  width  of  a  right-of-way  may  be  determined.  The  two  factors 
which  appear  to  have  had  the  greatest  influence  are  the  height 
of  adjoining  timber  and  the  probable  ultimate  number  of  pole 
or  tower  lines.  In  addition  to  these,  *there  are  several  other 
conditions  which  should  affect  the  width:  the  total  desired 
security  of^the  lines;  the  character  of  the  country  traversed;  and 
the  character  of  the  construction. 

Where  the  nature  of  the  ground  permits,  some  consideration 
should  be  given  to  patrol  or  transportation  facilities  between  the 
lines  of  supports.  Apart  from  any  question  of  cultivation  or 
possible  railway  facilities,  the  remaining  conditions  are  all  in- 
volved in  the  general  one  of  security  from  interruptions.  Inter- 
ruptions may  originate  either  within  or  without  the  limits  of  the 
right-of-way.  Those  from  within  are  generally  due  to  mechan- 
ical or  electrical  failure  and  are  best  minimized  by  separating 
the  lines.  Those  from  without — which  usually  exceed  the  former 
— include  falling  trees,  limbs,  straw  or  other  objects  blown  by 
the  wind,  fires,  malicious  mischief,  etc.  They  are  minimized  by 
moving  the  supports  in  from  the  side  lines.  The  two  types  of 


14  POLE  AND  TOWER  LINES 

interruption  are  therefore  prevented  by  opposite  action,  but  since 
the  latter  set  is  more  important  it  is  advisable  to  give  them  more 
weight  in  the  location  of  the  poles  or  towers.  It  has  sometimes 
been  stated  that  the  distance  from  the  side  lines  should  equal  or 
exceed  the  height  of  the  tallest  neighboring  trees.  Literally 
applied  this  rule  would  require  a  variable  width,  and  in  some 
localities  excessive  widths.  While  some  degree  of  consideration 
may  be  properly  given  to  the  average  height  of  timber,  it  must 
not  be  forgotten  that  the  effective  range  of  wind-blown  branches 
is  too  great  to  permit  absolute  protection.  It  is  advisable  to  cut 
down  or  trim  the  taller  trees  and  to  remove  dead  branches,  since 
storms  would  presumably  blow  these  onto  the  line  before  the 
smaller  and  live  timber  was  affected. 

The  character  of  the  construction  will  affect  the  separation 
of  the  supports  and  their  distance  from  the  property  lines  because 
the  wind-blown  sag  of  the  wires  must  be  given  proper  clearances, 
either  from  each  other  or  from  the  side  lines.  Long-span  con- 
struction will,  therefore,  require  greater  side  clearances  than 
short-span  construction.  Steel  poles  or  narrow-base  towers  per- 
mit the  closest  spacing  of  the  lines,  both  on  account  of  their 
narrow  spread  at  the  ground  and  because  they  are  usually  em- 
ployed with  shorter  spans  and  smaller  sags.  If  the  supports  are 
staggered,  i.e.,  the  poles  in  one  line  opposite  the  center  of  spans 
of  the  adjoining  line,  less  clearance  is  needed  to  prevent  swinging 
contacts.  The  general  security  desired  will  affect  the  width  of 
the  right-of-way  and  the  location  of  the  lines  thereon.  One  line 
in  the  middle  of  a  wide  right-of-way  has  the  maximum  possible 
security.  In  wild,  treeless  country  two  lines  near  the  edges  of 
the  property  are  more  immune  against  interruptions  than  with 
a  smaller  separation.  One  tall  and  one  low  line  are  more  im- 
mune than  two  tall  lines  because  the  lower  line  can  rarely  affect 
the  taller.  For  all  other  conditions,  the  lines  should  be  located 
so  as  to  permit  the  greatest  freedom  of  future  construction,  a 
reasonable  separation  between  lines,  and  a  maximum  clearance 
from  the  side  lines. 

In  order  to  facilitate  the  study  of  right-of-way  clearances, 
three  types  of  installation  are  shown.  Fig.  8  represents  a  two- 
circuit  steel  pole  or  narrow-base  tower  located  in  the  middle  of  a 
private  right-of-way;  Fig.  9  shows  two  one-circuit  poles,  while 
Fig.  10  shows  two  two-circuit  wide-base  towers.  It  is  assumed 
that  suspension  insulators  have  been  employed  in  each  case. 


GENERAL  CONSTRUCTION 


15 


FIG.  8. — Right-of-way — one  two-circuit  pole  line. 


FIG.  9. — Right-of-way — two  one-circuit  pole  lines. 


FIG.  10. — Right-of-way — two  two-circuit  tower  lines. 


16 


POLE  AND  TOWER  LINES 


To  modify  the  diagrams  for  pin  insulators  it  would  only  be  neces- 
sary to  adopt  a  slightly  smaller  value  for  the  wind-blown  deflec- 
tion B  and  for  the  tower  clearance  C.  The  windward-deflected 
sag  BI  in  Figs.  9  and  10  is  shown  as  one-half  that  of  the  leeward- 
deflected  sag  B.  This  assumption  is,  of  course,  arbitrary  and 
would  vary  for  different  sizes  of  wires,  spans  and  sags, 

In  Table  1  are  given  the  probable  ranges  of  the  various  clear- 
ances, the  summation  of  which  determines  the  width  of  the  right- 
of-way.  Also  shown  therein  is  the  probable  range  of  the  width 
W  with  three  assumed  minimum  side  clearances  A,  of  6  ft.,  15  ft., 
and  25  ft. 

The  presence  of  tall  trees  touching  the  right-of-way  line  is  un- 
desirable, but  it  cannot  always  be  avoided.  The  diagrams  repre- 
sent the  limiting  condition. 

TABLE  1. — APPROXIMATE  RANGE  OF  RIGHT-OF-WAY  CLEARANCES  (IN  FEET) 


One  double-cir- 
cuit pole  line 

Two  single-cir- 
cuit pole  lines 

Two  double-cir- 
cuit tower  lines 

Span 

300  to  500 

300  to  500 

500  to  800 

H  
B  

Bi 

20 
3  to  12 

20 
3  to  12 

1  to  6 

20 
10  to  25 

6  to  12 

C  
Pi 

3  to  6 

3  to  5 
3  to  5 

4  to    8 
3  to    6 

If  A  =  6  ft.,  W 
If  A  =  15  ft.,  W 
If  A  =  25  ft.,  W 


25  to  50 

40  to  65 
60  to  85 


35  to  80 
55  to  95 
75  to  120 


65  to  135 

85  to  155 

105  to  175 


Factor  of  Safety. — As  in  all  construction  work,  by  far  the  most 
important  determination  is  the  mechanical  factor  of  safety. 
Indeed,  the  factor  of  safety  will  not  only  narrow  the  selection  of 
the  details  of  construction,  but  will  practically  determine  the 
general  type  of  line  to  be  used.  In  past  practice,  however,  the 
factors  of  safety  have  frequently  been  the  last  values  to  be  defi- 
nitely determined,  whereas  they  should  be  the  basis  for  computa- 
tion. In  other  words,  the  method  in  general  use  is  a  cut-and-try 
method,  in  which  several  designs  having  in  reality  different  fac- 
tors are  first  worked  out,  and  the  selection  too  often  left  to  one 
individual's  judgment  or  to  the  suggestion  of  a  salesman.  It  is 
extremely  doubtful  whether  "competitive"  designs  received  by 


GENERAL  CONSTRUCTION  17 

most  purchasers  have  been  really  comparable,  in  so  far  as  their 
true  mechanical  factors  of  safety  were  concerned.  In  addition 
to  the  vagaries  of  competitive  bidding,  it  may  be  claimed  with 
considerable  justice  that  the  designs  ordinarily  made  by  a  pur- 
chaser are  not  truly  comparative.  For  an  accurate  survey  of  the 
conditions  influencing  the  selection  of  the  proper  factors  of  safety 
for  the  various  members  involved,  it  is  essential  that  considera- 
tion be  given  to  the  desired  length  of  service  of  the  line,  as. well  as 
to  the  characteristics  of  the  members  and  materials  involved  in 
the  construction.  Included  in  the  term  "length  of  service"  are 
many  indefinite  quantities  which  must  be  determined  by  judg- 
ment. Changes  in  the  capacity  of  the  line,  a  possible  increase 
in  voltage,  or  the  entire  elimination  of  the  line  from  an  operating 
standpoint  may  perhapfs  be  assumed  with  some  degree  of  ac- 
curacy, but  the  probable  life  of  the  materials  of  construction  and 
the  possibility  of  restrictive  public  regulation  are  extremely  diffi- 
cult to  determine,  either  for  a  given  line  or  for  future  develop- 
ments as  a  whole. 

The  factor  of  safety,  or  as  it  is  sometimes  termed,  "factor  of 
ignorance,"  is  a  much-abused  and  generally  misunderstood  ex- 
pression. In  reality  it  is  a  combination  of  the  allowances  for 
error,  and  consists  of  the  summation  of  the  individual  allowances 
or  elements  of  the  factor,  the  term  safety  being  somewhat  mis- 
leading. The  amount  of  the  total  factor  depends,  or  should 
depend,  on  the  accuracy  with  which  the  conditions  of  service 
and  the  characteristics  of  the  members  and  material  can  be  fore- 
told. If  the  possible  variation  of  all  the  individual  elements 
except  one  is  known,  then  the  allowances  for  the  known  elements 
entirely  eliminate  them  in  any  further  consideration  of  "safety," 
and  a  further  increase  in  the  total  factor  causes  a  disproportionate 
increase  in  the  one  unknown.  For  example,  the  factor  of  safety 
for.  wires  may  be  sub-divided  into  the  following  elements : 

(a)  Increased  loading. 

(6)  Uncertain  strength  of  the  material. 

(c)  Injuries  during  erection. 

(d)  Errors  in  erection  (improper  sag). 

(e)  Deterioration  in  the  material. 

While  the  theoretical  values  of  these  elements  should  vary  for 
each  installation,  the  actual  general  case  may  be  stated  as  ap- 
proximately : 


18  POLE  AND  TOWER  LINES 

a  =  0.30 
6  =  0.20 
c  =  0.10 
d  =  0.30 
e  =0.10 


Total  =1.00 
Breaking  strength  =  1 . 00 


Total  factor  of  safety  =  2 . 00 

In  other  words,  an  analysis  of  the  commonly  used  factor  of 
2.0  shows  that  it  permits  the  loading  to  be  underestimated  30 
per  cent.,  and  the  actual  breaking  strength  of  the  wire  20  per 
cent.,  and  allows  a  decrease  of  10  per  cent,  from  small  injuries, 
an  increase  in  stress  of  30  per  cent.,  due  to  improper  stringing, 
and  an  ultimate  deterioration  of  10  per  cent.,  before  the  span  is 
theoretically  at  the  point  of  failure. 

Omitting  for  the  moment  any  consideration  of  the  elastic  limit, 
and  the  fact  that  stresses  in  excess  thereof  will  necessitate  pulling 
up  slack  (with  or  without  other  undesirable  results),  it  is  evident 
that  any  further  increase  in  the  factor  of  safety  will  greatly  in- 
crease the  allowance  for  the  most  uncertain  element.  On  the 
other  hand,  if  the  strength  of  the  wires  assumed  in  the  design  cor- 
responds closely  with  the  material  as  purchased,  and  the  wires 
are  strung  with  care  and  with  sags  having  a  close  approximation 
to  those  in  the  design,  it  is  apparent  that  the  spans  will  safely 
withstand  a  very  considerable  increase  in  the  assumed  loading. 

A  little  consideration  of  the  probability  of  exceeding  the  above 
elements  in  a  line  designed  and  erected  with  reasonable  care  may 
explain  the  excellent  record  of  the  wires  in  existing  lines.  It  is 
more  than  probable  that  in  many  instances  inaccurate  wire 
stringing  has  entirely  changed  the  actual  strength  of  the  wires 
in  relation  to  external  loads,  and  as  indicated  by  the  allowance 
of  0.30  in  the  above  analysis,  this  is  the  most  uncertain  condition 
in  the  average  installation. 

The  above  analysis  is,  in  the  writer's  opinion,  a  fairly  accurate 
statement  of  the  average  actual  condition,  but  does  not  give  the 
correct  values  for  the  elements  of  the  factor  of  safety  of  wire 
spans  designed  and  constructed  under  competent  supervision. 

Spans. — Theoretically,  since  the  cost  of  the  material  between 
supports,  i.e.,  the  wire,  is  constant,  except  for  the  slight  increase 


GENERAL  CONSTRUCTION  19 

in  length  due  to  the  sag,  the  supports  should  be  spaced  far  apart. 
With  long  spans  the  number  of  insulators  is  reduced,  together 
with  the  probability  of  interruptions  originating  at  the  supports. 
However,  other  considerations  usually  prevent  the  adoption  of 
the  theoretically  economic  span  length.  The  conductors  must  be 
spaced  so  as  to  provide  sufficient  clearance  between  adjoining 
wires  and  between  the  wires  and  the  pole.  Therefore  with  an 
increase  in  span  length,  with  its  consequent  increase  in  sag,  it 
becomes  necessary  to  spread  the  wires  further  apart,  thus  length- 
ening the  crossarms  and  increasing  their  cost.  With  compara- 
tively few  wires  in  the  line,  it  is  possible  to  arrange  them  so  that 
long  spans  can  be  used  without  excessively  long  or  heavy  cross- 
arms,  but  on  heavy  lines  carrying  many  wires,  this  is  not  practi- 
cable without  unduly  increasing  the  height  of  the  poles. 

There  is  quite  a  difference  between  the  meaning  of  "  aver  age 
span"  and  " standard  span,"  the  former  being  the  final  result 
and  the  latter  the  original  design  which  provides  a  theoretical 
clearance  above  the  ground  in  flat  country.  Therefore,  unless 
the  towers  can  occupy  hill  tops,  or  an  extra  clearance  is  allowed 
in  the  design,  it  frequently  happens  that  intervening  elevations, 
or  the  loss  of  clearance  on  hillsides,  materially  decrease  the  actual 
span  length. 

If  lines  are  located  on  highways  long  spans  are  not  always 
practicable  as  the  length  of  crossarms  may  have  to  be  restricted, 
or  the  wooden  poles  available  may  be  incapable  of  withstanding 
the  load  due  to  long-span  construction. 

The  matter  is  further  complicated  by  the  mechanical  limita- 
tions of  standard,  or  stock,  crossarms,  pins,  and  insulators. 

On  steep  hills  the  spans  must  be  decreased,  or  the  supports 
lengthened,  to  maintain  the  overhead  clearance. 

The  size  of  the  conductors  has  more  effect  on  the  proper  or 
possible  length  of  span  than  any  other  condition,  since  the  large 
sags  required  for  small  wires  in  long  spans  would  necessitate 
excessive  wire  spacings  and  pole  heights. 

No  exact  economic  span  length  has  been  determined  either  for 
one  type  of  support  or  for  one  section  of  country.  In  fact,  it  is 
extremely  probable  that  for  any  particular  line  there  will  be  two 
designs  of  nearly  the  same  estimated  cost,  and  that  the  possible 
error  in  estimating  the  field  work  will  far  exceed  any  difference 
between  the  estimates  of  material. 

In  wood-pole  construction,  the  use  of  long  spans  with  high 


20  POLE  AND  TOWER  LINES 

poles  is  subject  to  a  serious  error  in  estimating  the  probable  re- 
placement cost.  In  view  of  the  recent  prices  and  scarcity  of 
long  poles,  it  may  be  possible  that  such  lines  cannot  be  rebuilt 
in  timber  at  any  reasonable  cost.  The  wood-pole  transmission 
line  is  an  entirely  proper  type  of  construction  in  many  cases,  and 
it  is  also  true  that  for  one  or  two  circuits  the  spans  could  often 
be  lengthened  with  advantage,  but  such  lines  should  be  protected 
from  decay  and  a  high  replacement  cost  used  in  estimating. 

While  the  economic  design  in  traversing  hilly  country  is  un- 
doubtedly to  cross  ravines  and  small  valleys  by  means  of  long 
spans,  it  is  possible  that  this  practice  may  be  injudicious  unless 
ample  clearance  is  provided  between  the  wires.  There  is  little 
exact  knowledge  of  the  dependence  to  be  placed  on  the  parallelism 
of  swinging  wires,  particularly  if  their  horizontal  spacing  is  only 
5  to  10  ft.  and  the  sag  15  to  30  ft.  Besides  the  accidental  contact 
of  wires  in  the  same  horizontal  plane,  there  have  been  instances 
of  the  lower  wires  being  lifted  by  the  wind  into  contact  with  those 
above. 

In  case  it  will  be  necessary  to  pay  rent  for  pole-rights  on  foreign 
property,  it  may  prove  economical  to  use  long-span  construction 
as  the  rent  saved  might  more  than  pay  interest  on  the  increased 
cost  of  the  supports. 

In  ordinary  country,  the  economic  span  is  probably  between 
400  and  500  ft.  for  narrow-base  supports,  and  between  600  and 
800  ft.  for  wide-base  towers. 

Supports. — The  poles  or  towers  used  up  to  the  present  time 
have  been  of  wood,  steel  and  reinforced  concrete,  and  they  have 
been  used  in  the  order  given,  both  as  to  numbers  and  priority  of 
installation.  Wood  poles,  still  the  most  common  form  of  support 
particularly  for  low- voltage  lines,  have  several  objectionable 
features  in  that  they  deteriorate  rather  rapidly,  do  not  resist  fire 
and  their  cost  is  increasing.  Under  certain  conditions,  however, 
wood  poles  are  still  economically  sound  construction  even  for 
high-voltage  lines,  although  the  time  is  not  far  distant  when 
they  will  no  longer  be  employed  for  first-class  installations. 

In  changing  from  wood  to  metal,  however,  we  may  profitably 
pause  to  consider  some  of  the  characteristics  of  the  structure 
which  has  rendered  possible  our  progress  in  line  construction. 
In  theory  as  well  as  in  fact  the  wooden  pole  is  a  precedent  for 
the  metal  structure,  and  a  too  violent  divergence  from  some  of 
its  good  features  may  result  in  structures  not  relatively  so  excel- 


GENERAL  CONSTRUCTION  21 

lent  as  the  wood  they  replace.  A  well-selected  timber  pole  is 
very  nearly  of  the  ideal  outline,  due  to  the  fact  that  the  stresses 
imposed  upon  it  in  its  original  life  were  almost  identical  in  nature 
with  those  encountered  in  pole-line  service.  It  should  not  be 
forgotten  that  a  wood  pole  has  equal  strength  in  all  directions, 
both  with  and  across  the  line,  and  a  comparatively  large  strength 
in  torsion.  These  qualities  tend  to  minimize  the  effect  of  acci- 


FIG.  11. — Bending   test,          FIG.  12. — Tower  of  the  rigid  or  windmill 
Coombs'  concrete  pole.  type. 

dental  loads  or  loads  other  than  those  assumed  in  design.  Again, 
wood  poles  have  considerable  elasticity  but  not  complete  flexi- 
bility, a  characteristic  which  enables  them  to  deflect  enough  to 
equalize  most  unbalanced  loadings  while  opposing  a  very  con- 
siderable restraining  force  against  the  spread  of  failures  along 
the  line.  This  semi-flexible  quality  of  wood  poles,  which  is  also 
obtainable  in  steel  or  reinforced  concrete,  is  probably  of  much 
greater  advantage  than  is  generally  realized.  Another  advantage 
of  wood  poles  is  that  they  are  not  easily  injured  in  handling,  and 
may  be  installed  by  men  of  ordinary  intelligence  and  training. 


22  POLE  AND  TOWER  LINES 

Moreover,  there  are  no  long  thin  sections  which  may  be  bent  and 
rendered  useless  and  no  flimsy  connections  in  the  make-up  of  a 
wooden  pole.  That  the  above  good  qualities  have  been  largely 
instrumental  in  securing  the  excellent  record  of  wooden  poles  in 
line  work  cannot  be  doubted  by  the  analyst,  and  their  lesson  is 
well  worth  attention. 

The  more  permanent  types  of  support  may  be  divided  into  the 
rigid  wide-base  steel  tower,  the  semi-flexible  pole  (either  of  steel 
or  reinforced  concrete),  and  the  flexible  steel  pole  or  frame. 
Apart  from  their  relative  cost  for  any  given  line,  there  is  to  be 
considered  the  ultimate  adaptability  of  each  type.  This  adapta- 
bility will  involve  the  questions  of  protective  coating,  rights-of- 
way,  freedom  from  serious  interruptions  to  service,  and  finally, 
and  to  the  writer's  mind  of  considerable  importance,  the  relative 
prominence  given  the  installation. 

In  the  progress  of  a  rapidly  growing  industry  there  is  always 
a  tendency  to  apply  methods  of  work  to  sections  of  the  country 
to  which  they  are  less  adapted  than  the  locality  of  their  previous 
successful  use.  The  adoption  of  the  so-called  wind-mill  tower 
may  not  be  judicious  in  the  densely  populated  districts  of  the  East 
where  climatic  conditions  are  severe.  In  such  regions,  there  are 
two  considerations  other  than  cost,  i.e.,  the  undue  prominence 
of  the  line  and  the  great  importance  of  failures.  The  wind-mill 
tower  is  rather  conspicuous,  but  it  provides  a  type  of  support 
with  which  failures  are  practically  confined  to  one  span.  Flexible- 
frame  supports  are  not  so  noticeable,  but  they  have  little  or  no 
strength  in  the  direction  of  the  line  and  will  therefore  presumably 
be  susceptible  to  a  more  severe  type  of  failure.  The  semi-flexible 
pole  or  tower,  occupying  a  position  about  midway  between  the 
two  types  of  structures  mentioned,  has  at  least  a  theoretical  ad- 
vantage over  either.  While  some  flexibility  is  useful  in  a  narrow- 
base  structure,  to  permit  "pull  back"  by  adjoining  span  wires, 
the  amount  of  deflection  need  not  be  excessive.  In  actual  service 
heretofore  this  movement  cannot  have  been  very  great,  for  the 
reason  that  the  commonly  used  attachments  have  not  sufficient 
strength  to  transmit  greatly  unbalanced  wire  tensions.  The 
desideratum  is  perhaps  a  certain  elasticity  rather  than  extreme 
flexibility.  In  fact,  moderate  bending  or  semi-flexibility  is  ob- 
tainable even  in  reinforced  concrete. 

In  sparsely  "settled  country  or  where  the  right-of-way  is  for 
any  reason  not  accessible  or  not  subject  to  cultivation,  the  spread 


GENERAL  CONSTRUCTION 


23 


of  tower  bases  is  unimportant.  If  more  than  one  high-voltage 
line  is  to  be  placed  upon  a  private  right-of-way,  the  separation 
of  the  lines  will  usually  depend  upon  factors  other  than  the  spread 
of  the  bases.  When  land  is  valuable  wide-base  towers  may  be 
impracticable.  For  instance,  there  will  probably  be  many  power 
lines  placed  upon  interurban  railway  rights-of-way.  Such  de- 
velopment is  natural  and  necessary,  but  railroad  rights-of-way 
do  not  provide  space  for  wide-base  construction. 


FIG.  13. — Flexible  A-frame. 


FIG.  14. — Semi-flexible  pole.' 


Wide-base  towers  and  semi-flexible  poles  should,  when  properly 
designed,  provide  the  maximum  security  against  interruptions 
to  service  caused  by  insulator  or  wire  failure.  The  greater 
strengths  attainable  in  such  structures  allow  the  use  of  longer 
spans,  with  a  consequent  reduction  in  the  number  of  insulators 
and  of  the  probability  of  insulator  failure.  In  case  of  wire  failure, 
whether  due  primarily  to  insulator  failure  or  not,  the  spread  of 


24 


POLE  AND  TOWER  LINES 


such  failure  along  the  line  is  arrested  before  it  has  influenced  more 
than  a  span  or  two.  The  remaining  factor  of  adaptability,  i.e., 
the  relative  prominence  of  the  various  structures  in  the  landscape, 
may  prove  of  considerable  subsequent  importance.  The  writer 
does  not  mean  to  imply  that  a  transmission  line  should  be  made 


FIG.   15. — River-crossing  tower. 

decorative,  but  rather  that  it  be  made  inconspicuous.  Even 
in  regard  to  decorative  effect,  it  is  not  absolutely  necessary  that 
it  be  an  ungainly  blot  upon  the  landscape.  Some  attention  to 
pleasing  outlines  is  not  amiss,  for  it  is  a  well-known  fact  that 


GENERAL  CONSTRUCTION 


25 


a  gracefully  designed  structure  is  usually  economical.  In  ad- 
dition to  the  question  of  appearance,  if  lines  situated  in  settled 
communities  are  to  remain  undisturbed  for  any  considerable 
period  of  time,  they  will  have  to  be  either  unobjectionable  in 
performance  or  invisible. 

The  design  of  steel  or  reinforced -concrete  poles  and  towers 
is  fortunately  becoming  less  hampered  by  demands  for  excessive 
cheapness,  and  the  regulations  current  in  other  structural  work 
are  no  longer  entirely  disregarded.  The  wisdom  of  this  should 
be  apparent  when  it  is  considered  that  a  few  hundreds  or  thou- 
sands of  dollars  " saved"  on  the  line  construction  may  jeopardize 


FIG.  16. — River-crossing  towers. 

the  efficiency  of  an  investment  of  millions  of  dollars.  It  is  true 
that  thus  far  existing  construction  has  given  fairly  satisfactory 
service,  but  it  is  equally  true  that  the  more  extended  use  of 
faulty  designs  would  eventually  bring  disrepute  upon  the 
industry,  and  through  failures  invite  the  enforcement  of  severe 
regulations  by  various  authorities.  In  any  type  of  support  the 
importance  of  eliminating  long  unsupported  members  and  of 
providing  a  firm  rigid  base  is  now  becoming  more  generally 
recognized. 

Location  Plan. — After  the  general  location  of  a  line  has  been 
determined  from  a  study  of  maps  and  inspection  of  the  ground, 
the  prompt  completion  of  the  location  plan  is  essential.  The 


26  POLE  AND  TOWER  LINES 

rapidity  of  the  compilation  of  this  plan  will  depend  on  whether 
the  line  is  to  occupy,  either  entirely  or  in  part,  a  strip  of  private 
right-of-way,  highways,  foreign  rights-of-way,  or  pole-rights. 
In  most  cases,  quite  accurate  preliminary  data  as  to  the  plan 
view  can  be  obtained  from  the  right-of-way  plans  of  properties 
such  as  steam  or  electric  railroads,  canals,  highways,  etc.  When 
a  private  right-of-way  has  not  been  entirely  secured,  some  changes 
in  alignment  may  be  expected,  so  the  location  plan  is  to  that 
extent  preliminary.  Except  for  the  desirability  of  having  a 
correct  permanent  record,  there  is  no  particular  need  of  determin- 
ing by  accurate  survey  the  exact  distances  between  widely 
separated  points. 

After  the  plan  has  been  brought  to  a  semi-final  stage,  the  profile 
should  be  drawn  upon  the  same  sheet.  In  doing  so,  considerable 
future  annoyance  may  be  avoided  by  drawing  a  true  profile 
and  breaking  the  view  at  the  corners,  so  that  corresponding 
points  will  occupy  their  correct  relative  position  in  plan  and 
profile,  and  both  the  center  line  and  datum  line  will  remain 
parallel  to  the  bottom  of  the  drawing.  In  some  cases  the 
distances  in  the  profile  have  been  measured  along  the  inclined 
surface  of  the  ground  and  then  plotted  horizontally,  which  results 
in  a  false  profile  and  compels  constant  reference  to  plan  and 
profile  to  identify  a  given  point.  It  does  not  make  a  particle  of 
difference  in  the  excellence  of  a  given  section  when  completed 
whether  the  distance  between  the  corners  is  4000  ft.  or  4050  ft. 
It  is  important,  however,  that  the  distance  from  small  steep  hills, 
etc.,  to  one  end  of  the  section  be  accurately  measured,  so  that  the 
poles  may  be  properly  located  to  give  the  required  clearance  over 
the  obstructions. 

The  tentative,  or  paper,  location  of  the  supports  can  now  be 
made  on  the  drawing  and  scrutinized  in  the  field  by  walking  over 
the  line.  It  is  assumed,  of  course,  that  in  the  preliminary  loca- 
tion reasonable  care  was  taken  to  avoid  natural  or  artificial 
obstructions  including  side  hills,  swamps,  flood  lands,  or  undue 
interference  with  other  lines,  and  the  use  of  private  property. 

Almost  invariably  minor  changes  will  have  to  be  made  to  fit 
the  paper  location  to  the  ground,  -and  local  surveys  can  be  made 
to  plot  cross-profiles  at  side  hills,  crossings,  encroachments,  etc. 
Some  supports  will  have  fixed  locations,  i.e.,  at  the  corners  in  the 
line,  etc.,  so  that  the  location  of  supports  is  reduced  to  distributing 
the  poles  or  towers  between  the  fixed  points — a  series  of  short 


GENERAL  CONSTRUCTION 


27 


locations.  In  a  line  having  many  changes  of  direction  and  eleva- 
tion, the  previously  assumed  standard  span  length  may  seldom 
be  used.  The  problem  is  then  one  of  ascertaining  the  economic 
or  desirable  span  length,  not  for  a  line  in  general,  but  for  a  given 
series  of  short  sections  having  fixed  ends  and  various  intervening 
hills  or  other  obstructions. 


400' 


17  250' 

FIG.  17. 


300' 


In  drawing  the  plan  and  profile,  it  will  usually  be  found  con- 
venient to  use  a  horizontal  scale  of  200  ft.  to  the  inch,  and  a  vertical 
scale  of  20  ft.  to  the  inch.  All  obstructions,  highways,  crossings, 
etc.,  should  be  located  in  plan,  and  shown  to  scale  in  the  profile. 

A  sag  and  clearance  templet  should  be  made  of  tracing  cloth, 
celluloid  or  even  of  thick  paper,  though  the  last  is  less  con- 
venient as  it  is  not  transparent.  Such  a  templet  is  shown  in 
Fig.  17.  The  curve  of  sag  is  formed  by  plotting  the  maximum 


28  POLE  AND  TOWER  LINES 

sags  of  the  given  wire  for  various  span  lengths  and  usually  for 
the  condition  of  high  temperature  and  no  ice  or  wind  load. 

Parallel  to  the  sag  curve,  and  at  the  distance  of  the  overhead 
clearance  below  it,  is  the  clearance  curve. 

The  templet  should  be  extended  to  include  span  lengths  well 
beyond  the  maximum  span  anticipated,  in  order  that  it  may  be 
used  on  hillsides. 

The  method  of  applying  the  templet  is  shown  in  Fig.  17,  in 
which  it  should  be  noted  that  the  base  of  the  templet  has  been 
kept  horizontal  and  the  templet  itself  shifted  until  the  sag  curve 
coincided  with  the  points  of  conductor  attachment  on  two 
supports,  without  causing  the  clearance  curve  to  intersect  the 
ground  line. 

In  case  the  standard  height  support  in  the  assumed  location 
will  not  permit  this,  the  span  must  be  decreased  or  the  support 
be  lengthened. 


CHAPTER  II 
LOADING 

No  detail  of  line  construction  has  been  the  subject  of  such 
inaccurate  assumptions  and  misstatement  of  facts  as  the  con- 
ditions of  loading  which  actual  existing  supports  should  or  would 
withstand.  Medium-voltage  lines,  located  in  regions  known  to 
be  subject  to  heavy  sleet,  have  been  described — and  apparently 
designed — on  the  assumption  that  no  sleet  load  would  occur,  and 
that  15  Ib.  wind  pressure  on  the  wires  and  30  Ib.  on  the  supports 
were  proper  assumptions.  It  has  been  stated  that,  in  some 
instances,  provision  has  been  made  for  the  supports  to  safely 
withstand  broken-wire  loadings  which  have  varied  from  one 
wire  to  one-half  or  two-thirds  the  number  of  wires.  Of  this 
entire  set  of  assumptions,  that  of  one  broken  wire  combined  with 
a  proper  wind  loading  is  often  a  more  accurate  statement  of  the 
actual  strength  of  the  existing  structures.  Fifteen  pounds  pressure 
on  a  No.  0  bare  wire  is  only  0.47  Ib.  per  linear  foot,  while  8.0  Ib. 
on  J^-in.  thickness  of  sleet  on  the  same  wire  is  0.91  Ib.  per  linear 
foot.  The  pressure  of  30  Ib.  per  square  foot,  on  the  tower 
corresponds  to  a  wind  velocity  of  112  miles  per  hour,  which  is  so 
excessive  as  to  provide  a  little  extra  strength  in  so  far  as  that  con- 
dition is  concerned.  The  maximum  tension  in  the  wires  would 
probably  be  about  2400  Ib.  per  wire,  and  the  usual  pin  insulators 
will  not  safely  withstand  such  stresses.  Furthermore,  very 
few  ties  or  clamps  will  dead-end  a  wire  under  such  tension. 
Again,  the  crossarms  used  in  certain  of  the  lines  under  discussion 
would  neither  carry  such  unbalanced  loads  nor  prevent  a  torsion 
which  at  less  load  would  permit  an  insulator  to  incline  and  the 
wire  to  pull  free.  It  is,  therefore,  evident  that  the  assumed 
broken-wire  stress  could  not  be  transmitted  to  the  support. 

The  writer  believes  that  no  consideration  need  be  given 
accidental  loads  caused  by  falling  objects  such  as  trees,  etc.,  and 
that  a  single  ice  and  wind  load  will  apply  very  satisfactorily  in 
nearly  every  part  of  this  country.  It  is  true  that  in  certain 
localities  either  a  smaller  or  a  larger  loading  may  be  justifiable, 

29 


30 


POLE  AND  TOWER  LINES 


and  that  some  installations  may  warrant  greater  security  than 
others,  but  these  are  questions  for  engineering  judgment  and 
should  not  influence  general  construction. 


FIG.  18. — Snow-ice  loads. 


Sleet. — As  it  is  hardly  practicable  to  attempt  the  consideration 
of  accidental  loads  which  can  be  caused  by  falling  objects,  the 
only  external  loads  to  be  considered  are  the  ice  and  wind  loads 


FIG.  19. 


on  the  wires  and  their  supports.  Severe  loads  of  this  nature 
are  rare,  and  those  producing  very  excessive  stresses  may  be 
regarded  as  being  in  the  category  with  tornadoes  and  similar 


LOADING 


31 


visitations  which  are  beyond  the  limits  of  design.  It  has  been 
shown  by  the  records  of  the  telephone  companies,  and  is  now 
more  generally  understood,  that  sleet  loads  may  be  encountered 
throughout  nearly  the  entire  United  States,  with  the  possible 
exception  of  certain  restricted  localities  in  the  South  and  West. 

The  maximum  amount  of  sleet  undoubtedly  varies,  but  the 
effective  variation  of  the  combined  wind  and  ice  load  is  much 
less  than  is  generally  believed.     Further,  and  neglecting  the  oc- 
casional extremely  heavy  deposits,  it  seems 
probable  that  a  maximum  thickness  of  1 
in.  may  be  encountered.     Experience  has 
shown,  contrary  to  the  earlier  assumption 
of  many  engineers,  that  sleet  deposits  will 
occur  on  wires   carrying  voltages  up  to 
60,000  and  possibly  much  higher. 

The  heavier  deposits  are  often  of  snow- 
ice  and  of  less  weight  than  clear  ice,  be- 
sides being  more  subject  to  removal  by 
the  sun  and  wind.  Ice  deposits,  on  the 
other  hand,  often  remain  intact  even  under 
a  bright  winter  sun  and  a  rising  wind. 

It  is  extremely  improbable  that  every 
span  in  any  given  line  will  ever  be  sub- 
jected to  the  simultaneous  action  of  the 
maximum  sleet  and  wind  loads.  The 
maximum  sleet  load,  provided  for  in  the 
design,  is  in  itself  a  rare  occurrence  for 
any  given  span,  perhaps  happening  once 
in  ten  years.  Moreover,  it  is  assumed 
that  the  sleet  remains  in  place  throughout  the  span  and  that  the 
wind  rises  to  a  velocity  which  in  itself  should  occur  but  two  or 
three  times  each  year  during  the  winter  months.  It  has  some- 
times been  specified  that  the  thickness  of  sleet  sho'uld  be  a  factor 
of  the  diameter  of  the  wire — an  assumption  which  is  not  borne 
out  by  the  facts.  Indeed  the  effect  of  the  sleet  load  is  far  greater 
upon  small  wires,  since  their  area — and  strength — is  much  smaller, 
while  the  wind  and  sleet  loading  is  only  a  little  less  than  for 
larger  wires. 

The  following  were  reported,  in  1915,  as  being  the  maximum 
loadings  to  which  nine  tower  lines  had  actually  been  subjected 
in  service,  without  injury.  It  should  be  noted  that  while  the 


FIG.  20. 


32  POLE  AND  TOWER  LINES 

sleet  load  was  usually  measured,  the  wind  load  was  not  known 
and  not  reported. 

No.  1 no  sleet,  2  in.  thickness  of  snow. 

No.  2 2  in.  thickness  of  snow. 

No.  3.  . Y±  in.  thickness  of  sleet  and  2*4  in.  of  soft  snow. 

No.  4 %  in.  thickness  of  sleet. 

No.  5 %  in.  thickness  of  sleet — wind  35  miles. 

No.  6 1  in.  thickness  of  sleet. 

No.  7 2  in.  thickness  of  sleet. 

No.  8 2    cables  parted  during  construction. 

No.  9 1    cable  parted  during  construction. 

The  most  reasonable  assumption  for  general  use  in  regions 
where  sleet  is  known  to  occur  would  seem  to  be  a  thickness  of 
%  in.  all  around  the  wires,  combined  with  a  wind  load  which 
will  be  discussed  in  the  following  section. 

Wind. — It  has  been  stated  by  some  writers  that  it  is  necessary 
to  know  the  probable  direction  of  the  wind  and  whether  the 
wind  and  ice  loads  may  occur  together  before  the  line  may  be 
designed. 

In  ordinary  broken  country  and  with  the  usual  changes  in 
direction  of  a  line,  the  former  information,  even  if  obtainable, 
could  hardly  be  of  great  service.  For  example,  it  would  be  very 
bad  practice  to  assume  that  the  wind  would  blow  in  but  one 
direction,  and  to  use  a  structure  incapable  of  resisting  pressure 
from  the  opposite  direction. 

The  second  condition  mentioned  contains  a  fallacious  assump- 
tion in  that  it  might  be  inferred  that  high  winds  and  sleet  will  not 
occur  together.  For  the  vast  majority  of  transmission  lines  in 
this  country  the  sleet  load,  if  there  is  ever  sleet  in  the  section  in 
question,  will  probably  occur  during  months  in  which  high  winds 
also  occur.  Again,  it  has  frequently  been  claimed  that  high 
wind  loads  occurring  during  warm  weather  exert  a  greater  effect 
upon  the  wires  and  their  supports  than  the  combined  sleet  and 
wind  loads  of  the  winter  months.  Disregarding  cyclonic  storms, 
which  in  many  instances  are  beyond  the  limits  of  reasonable 
design,  the  writer  believes  the  above  claim  to  be  entirely  false 
and  dangerously  misleading.  According  to  reports  from  the 
United  States  Weather  Bureau,  the  maximum  recorded  wind 
pressures  in  many  localities  have  occurred  during  the  winter 
months.  The  combination  of  sleet  and  a  moderately  low  wind 
velocity  is  greater  in  effect  than  the  highest  warm-weather  wind 
pressures. 


LOADING 


33 


The  Joint  Report  Specifications  for  crossings  require  a  figured 
loading  of  %  in.  thickness  of  ice  and  8  Ib.  per  square  foot  wind 
pressure  on  the  ice-covered  diameter  of  the  wires.  This  loading 
was  considered  by  the  framers  of  the  specifications  as  being 
generally  reasonable,  and  with  the  designated  factor  of  safety, 


FIG.  21. — Conductors  deflected  by  high  wind. 

etc.,  to  provide  the  proper  construction  for  a  crossing.  It  is 
not  denied  that  thickness  of  ice  greater  than  0.5  in.  or  pressures 
of  wind  greater  than  8  Ib.  may  occur,  but  it  is  improbable  that 
they  will  occur  simultaneously  over  large  areas  or  so  frequently 
as  to  make  it  desirable  to  impose  a  greater  loading  on  all  future 


34 


POLE  AND  TOWER  LINES 


crossings.  All  of  the  spans  in  a  given  line  would  never  be 
subjected  to  the  maximum  figured  load,  nor  would  a  number  of 
adjacent  short  spans,  or  even  one  very  long  span,  be  likely  to 
receive  the  maximum  load  over  every  lineal  foot.  It  is  true  that 
telephone  lines  fail  every  winter,  and  perhaps  that  some  old  or 
incorrectly  built  low-voltage  lines  occasionally  fall,  but  the  writer, 
has  yet  to  learn  of  the  failure  of  a  single  wire  strung,  even  ap- 
proximately, to  the  Joint  Report  requirements.  Further  it  should 


16 


12 


10 


•sX 


200 


500 


300  400 

Span  in  Feet 

FIG.  22.  —  Comparative  normal  sags  of  No.  1  wire  for  various  loadings. 

be  remembered  that  the  loading  and  factors  of  safety  in  question 
—  and  these  must  be  considered  in  conjunction  —  were  recom- 
mended for  crossings  and  for  crossings  only.  As  yet  they  have 
not  been  recommended  by  any  authoritative  body  for  general 
intermediate  line  construction. 

Some  specifications  have  provided  for  a  load  of  0.25  in.  of  ice 
and  8  Ib.  per  square  foot  wind  pressure  with  a  stress  limit  of 
0.9  of  the  elastic  limit  of  the  wire,  while  at  least  one  crossing 
specification  contains  the  severe  requirement  of  0.5  in.  of  ice 
and  20  Ib.  per  square  foot  wind  pressure  with  a  stress  limit  of 


LOADING 


35 


0.4  of  the  ultimate  strength  of  the  wire.  In  order  to  indicate 
more  clearly  the  relative  effect  of  various  loadings  and  factors 
of  safety,  the  approximate  curves  in  Fig.  22  have  been  prepared 
to  show  the  normal  sag  (at  60°F.  unloaded)  of  a  No.  1,  B.  &  S. 
gage,  hard-drawn  stranded  copper  wire  under  the  following  condi- 
tions : 

(1) ...  0 . 5  in.  ice  +  20 . 0  Ib.  wind  (120  miles  per  hour")  max.  stress,  0 . 4  ultimate  =  1580  Ib. 
(2) ...  1 . 0  in.  ice  -j-  2.8  Ib.  wind  (  40  miles  per  ho.ur)  max.  stress,  0 .  5  ultimate  =  1980  Ib. 

(3)  ...  0 . 5   in.  ice    -j-    8.0  Ib.  wind  (  70  miles  per  hour)  max.  stress,  0 .  5  ultimate  =  1980  Ib. 

(4)  ...  0 . 5    in.  ice    +    8.0  Ib.  wind  (  70  miles  per  hour)  max.  stress,  0 .  6  ultimate  =  2370  Ib. 
(5) . .  .0.25  in.  ice   +    8.0  Ib.  wind  (  70  miles  per  hour)  max.  stress,  0.9  elastic       =2110  Ib. 

The  records  of  the  United  States  Weather  Bureau — omitting 
tornadoes,  cyclones,  and  violent  gales  occurring  in  some  par- 
ticularly exposed  localities — show  a  maximum  indicated  velocity 
of  100  miles  per  hour.  The  records  at  Bidston  Observatory, 
Liverpool,  England,  covering  the  period  from  1884  to  1888,  give 
an  actual  velocity  of  78  miles  per  hour  as  a  maximum  of  10 
severe  storms.1 

Table  2  shows  the  maximum  velocities  observed  at  a  number 
of  stations  by  the  United  States  Weather  Bureau. 

TABLE  2. — MAXIMUM  WIND  VELOCITIES 


Observatory 

Period 

Maximum 
velocity 
indicated 

Observatory 

Period 

Maximum 
velocity 
indicated 

Chicago    111 

1871-1906 

90 

Savannah    Ga 

1894-1903 

76 

Buffalo,  N   Y 

1871-1907 

90 

Philadelphia,  Pa 

1872-1907 

75 

Galveston,  Tex  
New  York,  N.  Y  
Eastport,  Me  

1894-1903 
1871-1907 
1873-1907 

84 
80 

78 

Bismarck,  N.  Dak  
Boston,  Mass  
Salt  Lake  City,  Utah 

1894-1903 
1873-1907 
1894-1903 

72 
72 
60 

Table  3  shows  the  three  highest  indicated  velocities  recorded 
each  year  by  the  United  States  Weather  Bureau  in  its  New  York 
City  station,  during  the  period  from  1884  to  1906  inclusive.  This 
station  was  moved  in  March,  1895,  from  the  Manhattan  Life 
Insurance  Building  to  the  location  at  100  Broadway;  the  latter 
is  evidently  in  a  more  exposed  position,  as  shown  by  the  abrupt 
rise  in, velocities  after  1895.  The  maximum  velocity  of  80  miles 
per  hour  occurred  during  a  sleet  storm. 

1  Extract  from  Overhead  Construction  for  High-tension  Electric  Traction 
or  Transmission,  by  R.  D.  Coombs,  Transactions  of  American  Society  of 
Civil  Engineer,  Vol.  LX. 


36 


POLE  AND  TOWER  LINES 


TABLE  3. — RECORD  OF  HIGHEST  WIND  VELOCITIES  IN  NEW  YORK  CITY 


Year 

Date 

Maximum 
velocity 

Date 

Maximum 
velocity 

Date 

Maximum  • 
velocity 

1884 

Oct.    18 

44 

Feb.  20 

40 

Dec.     9 

40 

5 

Jan.    17 

50 

Dec.     7 

50 

Mar.  10 

48 

6 

Feb.  26 

64 

Mar.    2 

54 

Jan.      9 

44 

7 

Dec.  29 

50 

Nov.  16 

48 

Feb.   12 

46 

8 

Jan.    26 

60 

Mar.    5 

'    52      . 

Mar.  13 

50 

9 

Jan.    17 

50 

Feb.     1 

48 

Dec.  26 

48 

1890 

Jan.    22 

55 

Dec.  17 

48 

Feb.     5 

45 

1 

Dec.  30 

53 

Mar.  14 

45 

Jan.    11 

44 

2 

Jan.    26 

49 

Mar.  11 

40 

Jan.     5 

40 

3 

Aug.  29 

54 

Jan.      1 

48 

Oct.    13 

48 

4 

Apr.   11 

48 

Oct.    10 

48 

Jan.    12 

43 

5 

Dec.  27 

73 

Mar.  28 

64 

Aug.     4 

62 

6 

Mar.    4 

72 

Feb.     7 

65 

Sept.  30 

5fr 

7 

Jan.    18 

60 

Feb.     6 

60 

Oct.    17 

60 

8 

Dec.     4 

78 

Sept.    7 

72 

Nov.  11 

65 

9 

Mar.  20 

80 

Jan.   25 

66 

Feb.  27 

64 

1900 

Oct.    16 

76 

Nov.  21 

76 

Jan.    26 

76 

1 

Nov.  26 

72 

Jan.    19 

72 

Feb.     5 

70 

2 

Mar.  19 

74 

Jan.      1 

74 

Feb.     2 

74 

3 

July     2 

72 

Feb.     5 

72 

Sept.  17 

65 

4 

Apr.    16 

73 

Sept.  15 

68 

Mar.    3 

65 

5 

Dec.   10 

64 

Feb.     7 

61 

Apr.   10 

56 

6 

Mar.  10 

64 

Jan.      6 

61 

!  Feb.  28 

59 

Table  4  is  a  record,  by  months,  of  the  number  of  different 
12-hour  periods  during  which  a  maximum  velocity  of  60  miles, 
or  more,  was  observed  at  the  New  York  City  station  from  1895 
to  1906  inclusive.  Inasmuch  as  a  maximum  occurring  late  in 
one  period  and  another  early  in  the  following  period  are  both 
entered,  a  few  of  the  entries  represent  the  effects  of  the  same 
storm. 

For  the  vicinity  of  New  York  City,  Tables  3  and  4  indicate 
that:  the  maximum  velocities  occur  during  the  winter  months, 
when  sleet  may  be  on  the  wires;  indicated  velocities  of  more 
than  80  miles  per  hour  will  rarely,  if  ever,  occur  during  the  life 
of  a  given  structure;  and  indicated  velocities  of  65  to  75  miles 
per  hour  may  be  expected  several  times  each  year,  though  much 
less  frequently  in  conjunction  with  sleet. 

A  rather  complete  study  and  tabulation  of  the  U.  S.  Weather 
Bureau  records  of  43  observatories,  located  in  32  States,  and 


LOADING 


37 


TABLE  4. — NUMBER  OF  12-nouR  PERIODS  IN  WHICH  WIND  VELOCITIES  OF 
60  MILES  PER  HOUR  OR  HIGHER  WERE  OBSERVED  IN  NEW  YORK  CITY 


Month 

Indicated  velocities,  in  miles  per  hour 

Totals 

60 

61 

62 

63 

64 

65 

66 

67 

68 

70 

72 

73 

74 

76 

78 

80 

Jan 

3 
7 
3 

3 
2 

2 

1 

•1 
1 
2 
1 

2 
6 
2 

1 
2 
1 

4 
2 
1 

1 

1 

1 

18 
31 
17 
3 
3 
3 
4 
3 
6 
3 
13 
18 

Feb 

2 

2 

1 

3 
1 

1 
3 

1 

1 
1 

1 

Mar 

Aur 

1 

May 

1 

1 

1 

1 

1 

1 

July 

9 

1 

1 

1 

1 

1 

Sent 

1 
2 

1 

2 
3 

2 

1 
1 

1 

1 

Oct 

1 
4 
6 

1 

1 

2 
1 

, 

1 

1 
1 

1 
1 

1 

1 
1 

1 

Nov  

Dec     

Totals  

26 

8 

8 

9 

18 

10 

9 

2 

5 

4 

10 

3 

4 

4 

1 

1 

122 

covering  periods  of  observation  from  5  to  43  years  results  in  the 
following  summary1: — 


Total  number  of  sleet  storms 487 

Number  with  wind  velocity  over  40  miles  per  hour.  ....     31 

Number  with  wind  velocity  over  50  miles  per  hour 12 

Number  with  wind  velocity  over  60  miles  per  hour 5 

Sleet  formation,  ^  in.  or  less,  during  90  storms 
Sleet  formation,  ^  in.  to  ^  in.  during  62  storms 
Sleet  formation,  %  in.  to  1  in.  during  42  storms 
Sleet  formation,  over  1  in.  during  17  storms 

Total  recorded   =211 

Temperature  fell  below  0°  F.,  after  sleet  deposit ....  3  storms 
Maximum  recorded  wind  velocity  and  maximum 
sleet  deposit  occurring  in  same  storm 19  storms 

Since  the  publication  of  Sir  Isaac  Newton's  law  for  the  pressures 
exerted  by  moving  fluids — which,  for  wind  pressures,  may  be 
reduced  to  the  form 

P  =  —  F2 
370 

in  which  P  =  pressure,  in  pounds  per  square  foot,  and   V  = 
velocity,  in  miles  per  hour— many  investigators   have   experi- 
mented, with  a  view  to  the   determination   of   values   for   the 
1  Handbook  on  Overhead  Line  Construction,  National  Elec.  Light  Assoc. 


38 


POLE  AND  TOWER  LINES 


constant,  K.     For  normal  pressures  against  thin  flat  surfaces, 
most  of  the  results  indicate  values  between 


and 


P  =  0.0035F2 
P  =  0.0049  72 


(1) 
(2) 


These   formulas,   modified   to   apply   to   cylindrical   surfaces, 
become 

P  =  0.0021 72  (3) 


and 


P  =  0.0029  V2 


(4) 


The  Berlin-Zossen  high-speed  tests,  in  which  wind  pressures 
against  trains  were  measured,  gave  the  formula, 

P  =  0.0027  F2 

and,  using  a  rounded  "nose"  on  the  forward  end, 

P  =  0.0025  V2 

In  Table  5  are  given  the  equivalent  actual  velocities  corre- 
sponding to  those  indicated  by  anemometer  readings,  and  the 
pressures  per  square  foot  produced  on  flat  and  cylindrical 
surfaces. 

TABLE  5. — WIND  PRESSURES  AND  VELOCITIES  CORRESPONDING  TO  ANEMOM- 
ETER READINGS 


Indicated  velocities, 
mi.  per  hr. 

Actual  velocities, 
mi.  per  hr. 

Pressure  per  sq.  ft.  on 
cylinders, 
P  =  0.0025F2 

Pressure  per  sq.  ft.  on 
flat  surfaces, 
P  =  0.004272 

30 

25.7 

1.7 

2.8 

40 

33.3 

2.8 

4.6 

50 

40.8 

4.2 

7.0 

60 

48.0 

5.8 

9.7 

70 

55.2 

7.6 

12.8 

80 

62.2 

9.7 

16.2 

90 

69.2 

12.0 

20.1 

100 

76.2 

14.6 

23.3 

110 

83.2 

17.3 

29.1 

120 

90.2 

20.3 

34.2 

P  =  pressure,  in  pounds  per  square  foot. 
V  =  velocity  (actual),  in  miles  per  hour. 

Assuming  an  indicated  velocity  of  70  miles  per  hour,  or  an 
actual  velocity  of  55.2  miles  per  hour,  the  above  equation  for 


LOADING  39 

obtaining  pressures  against  flat  surfaces  becomes  P  =  12.8  Ib. 
per  square  foot  of  projected  area,  while  for  pressures  against 
cylindrical  surfaces  P  =  7.6  Ib.  per  square  foot  of  projected  area. 
On  long  spans,  the  maximum  pressure  at  one  point  may  be 
considerably  in  excess  of  the  equivalent  uniform  pressure  along 
the  wire,  while  very  short  spans  may  be  exposed  to  the  maxi- 
mum pressure  throughout  their  length.  In  view  of  the  rare,  if 
not  improbable,  occurrence  of  indicated  velocities  greater  than 
80  miles  per  hour,  and  the  further  improbability  of  such  winds 
accompanying  sleet  storms,  or  of  the  sleet  remaining  in  place, 
the  following  pressures  seem  to  be  reasonable  for  general  use: 

P  =  13.0  Ib.  per  square  foot  of  projected  area  for  flat  surfaces; 
P  =    8.01b.  per  square  foot  of  projected  area  of  wires  covered 
with  0.5-in.  deposit  of  ice. 

In  applying  wind  loads  to  the  supports,  wooden,  concrete  or 
cylindrical  metal  poles  should  be  considered  as  flat  surfaces. 
By  so  doing  the  excess  loading  will  compensate  for  the  increased 
surface  at  the  top  of  the  poles  due  to  arms,  braces,  insulators,  etc. 
Latticed  steel  poles  or  steel  towers  should  be  treated  as  having 
flat  surfaces  equal  to  the  exposed  area  of  the  members  on  the 
windward  side,  increased  by  50  per  cent,  to  allow  for  pressure  on 
the  leeward  side  of  the  poles  and  by  100  per  cent,  for  wide-base 
towers. 

On  the  other  hand  some  decreases  in  pressure  would  be  justi- 
fied on  the  lower  part  of  poles  or  towers,  except  when  set  on 
hill-tops. 

Broken  Wires. — In  determining  the  proper  wire  loads  and 
factors  of  safety  to  be  used,  it  is  important  to  bear  in  mind  the 
effect  of  any  further  requirement  such  as  a  provision  for  dead- 
ending  or  carrying  broken  wires,  inasmuch  as  the  effect  of  the 
latter  requirement  is  to  impose  from  5  to  40  times  the  former  load- 
ing upon  the  insulator  connections  and  the  supporting  structures. 
Dead-ending  and  corner-turning  are  different  only  in  degree, 
and  designing  for  a  broken-wire  load  is  more  or  less  equivalent  to 
designing  all  structures  as  corner  structures. 

Fig.  23  is  a  graphical  representation  of  the  relative  effect  of 
what  is  termed  balanced  transverse  loading  and  a  broken-wire 
condition.  The  ordinates  are  the  ratios  of  loads  caused  by  one 
broken  wire  to  the  loads  caused  by  balanced  spans.  In  other 
words,  the  ordinates  show  how  many  times  more  severe  a  broken- 


40 


POLE  AND  TOWER  LINES 


wire  condition  is  than  the  load  of  the  same  wire  unbroken  under 
identical  ice  and  wind  loads.  For  instance,  a  No.  1  hard-drawn 
stranded  copper  wire  in  spans  of  200  ft.  would,  for  a  broken- wire 
condition,  impose  16  times  the  stress  upon  its  support  that  it 
would  unbroken. 


40 
38 
36 
.     34 
32 
30 

"g   26 

CI 

|    24 
3    22 

1   20 

h    13 

£ 
g    16 

'S    14 

'S    12 
•2 

d  10 

8 
6 
4 
2 

o, 
FIG. 

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\ 

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\ 

$1 

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0*$^ 

>od 



— 

\ 

% 

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\ 

<*> 

% 

\ 

X 

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^ 

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b 

^X 

^ 

x^ 

\ 

^ 

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^^ 

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°lt<1 

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.. 

—   — 

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15            100                               150                              200                             2SO 
Span  in  Feet 
23.  —  Relative  effect  of  balanced  and  broken  wire  loads. 

In  determining  a  mechanical  factor  of  safety  for  any  material 
it  is  customary,  whether  so  stated  or  not,  to  assume  certain  por- 
tions of  the  factor  as  safeguarding  each  of  the  possible  elements 
of  danger,  such  as  errors  of  design,  workmanship,  excess  loads 
and  deterioration  of  material.  When  using  wire  cables  a  rel- 
atively low  factor  may  be  assumed,  since  a  wire  catenary,  both 
in  material  and  as  a  structural  member,  is  more  uniform  in  sec- 
tion, strength  and  elasticity,  and  less  influenced  by  eccentric 


LOADING  41 

loads  or  errors  of  workmanship,  than  any  other  engineer- 
ing structure.  Therefore,  failure  in  the  wires  may  be  con- 
sidered as  resulting  usually  from  electrical  causes  such  as 
arcs  in  the  span  or  at  the  insulators.  Assuming  the  provision 
of  adequate  clearance  and  proper  spacing  of  wires  in  the  span, 
the  majority  of  wire  troubles  should  occur  at  the  insulators. 
Further,  and  in  view  of  the  small  number  of  failures  per  in- 
sulator in  the  existing  installations,  it  would  seem  that  the  in- 
creasing tendency  to  improve  the  insulation  should  have  some 
effect  in  lowering  the  number  of  broken  wires  per  support. 

In  consideration  of  the  above  it  is,  in  the  writer's  opinion  at 
least,  utterly  indefensible  to  assume  a  severe  broken-wire  con- 
dition in  designing  all  poles  and  towers.  Particularly  is  this  true 
if  future  construction  must  actually  accord  with  specifications. 
There  can  be  no  engineering  justification  for  a  specification 
which  premises  a  large  proportion  of  wires  broken  under  full 
load,  when  the  devices  fastening  the  conductors  to  the  sup- 
ports would  not  withstand  any  considerable  part  of  such  a  load. 
Again,  it  is  probably  a  fact  that  many  of  the  structures  in  exist- 
ing lines  are  not  as  strong  as  the  preliminary  test  structure.  This 
may  be  due  to  a  variety  of  causes,  such  as  local  injury  or  in- 
cipient bends  in  light  sections,  lack  of  rigidity  in  the  founda- 
tions, weakness  in  torsion,  and  last  but  not  least,  the  usual 
difference  between  test  specimens  and  the  least  perfect  field 
product. 

In  working  backward  from  the  results  of  practical  expe- 
rience over  large  areas,  the  tendency  is  to  overestimate  both  the 
actual  loading  and  the  strength  of  the  structures.  In  other 
words,  many  existing  lines,  particularly  the  heavy  wooden  pole 
lines,  remain  in  service  without  failing  not  because  they  have  a 
strength  equivalent  to  some  recent  requirements,  but  simply 
because  they  have  never  been  subjected  to  such  loads.  There- 
fore if  a  severe  mechanical  requirement  is  placed  in  a  standard 
specification  it  must  be  assumed  that  designers  will  eventually 
be  driven  to  literal  compliance  therewith,  and  the  net  result  will 
not  be  equivalent  to  the  designs  of  the  transition  period  upon 
which  the  requirement  is  supposed  to  be  founded. 

The  assumed  load  on  the  wires  should  equal  as  nearly  as 
possible  the  maximum  load  that  may  be  expected  on  some 
indeterminate  number  of  spans  during  some  indeterminate 
interval  of  time.  One  or  more  spans  in  a  given  line  may  con- 


42  POLE  AND  TOWER  LINES 

ceivably,  and  properly,  receive  a  greater  load  during  their  life- 
time. Such  excess  loads  may,  or  may  not,  be  harmful,  depend- 
ing on  the  factor  of  safety.  A  large  factor  of  safety  will  un- 
doubtedly continue  to  protect  inaccurate  assumptions  of  load- 
ing, but  the  use  of  unreasonable  loads  and  impossible  stresses 
does  not  establish  wise  engineering  standards. 

Bridges  and  buildings  are  not  designed  to  withstand  tornadoes, 
nor  need  power  wires  be  absolutely  immune  from  failure.  It 
is,  however,  becoming  more  and  more  important  to  provide 
continuous  service  and  to  establish  a  standard  which  will  satisfy 
all  conflicting  interests  without  unduly  burdening  a  great  industry. 

The  difficulty  in  specifying  a  sliding  scale  of  broken  wires  is 
that  in  reality  only  one  or  two  wires  may  be  reasonably  ex- 
pected to  break,  whether  there  is  one  circuit  upon  the  structure 
or  one  dozen  circuits.  If  there  are  only  three  wires  on  a  pole, 
the  load  requirement  of  one  broken  wire  is  relatively  greater 
than  the  requirement  of  several  broken  wires  on  a  structure 
of  many  wires  on  account  of  the  pullback  of  adjoining  spans. 

Again,  if  broken  wires  are  to  be  considered,  the  wire  con- 
nections must  be  designed  to  withstand  a  broken-wire  loading, 
otherwise  the  broken-wire  load  could  not  be  transmitted  to  the 
support. 

To  allow  pullback  in  a  consistently  designed  line  is  correct 
both  theoretically  and  practically,  but  its  accurate  computation 
is  quite  difficult,  and  the  inclusion  of  such  a  condition  in  a 
general  specification  is  probably  inadvisable.  It  appears,  there- 
fore, that  a  broken-wire  load  should  be  applied  to  the  arms 
and  wire  connections,  but  that  its  application  to  the  supports 
may  depend  in  a  measure  upon  the  character  of  the  supports. 


CHAPTER  III 
WIRES  AND  CABLES 

The  qualities  desired  in  electric-service  wires,  in  so  far  as  the 
construction  is  concerned,  are  mechanical  strength,  tenacity,  and 
ability  to  resist  corrosion  or  other  deterioration.  In  ordinary 
practice,  the  breaking  strength  required  for  a  wire  of  a  given 
span  will  depend  entirely  on  the  sag,  because  increasing  the  sag 
will  reduce  the  wire  tension  approximately  in  proportion  to  the 
sag.  Practical  considerations,  however,  indicate  a  rather  in- 
definite minimum,  below  which  it  is  undesirable  to  go.  Any 
surface  injury,  such  as  local  pitting  by  arcs  and  nicks  caused 
by  careless  handling,  or  any  weakness  in  the  material  due  to 
errors  in  manufacture,  will  have  relatively  greater  effect  on  a  small 
wire  than  on  a  large  one.  Moreover,  such  faults  are  more 
serious  in  solid  wires  than  in  stranded  cables,  and  in  hard-drawn 
wire  than  in  soft-drawn  wire.  In  stranded  cables,  an  injury  to 
a  single  strand  affects  only  a  fractional  part  of  the  entire  sec- 
tion; in  hard-drawn  wire  the  surface,  or  skin  material,  has  ap- 
proximately twice  the  unit  strength  of  the  interior  mass,  so  that 
an  injury  will  have  a  relatively  greater  effect  on  hard-drawn 
wire.  Fortunately,  however,  the  process  of  wire  drawing  in- 
sures a  large  amount  of  work  per  unit  of  mass,  so  that  the  finished 
product  is  a  very  homogeneous  and  trustworthy  material. 
This  quality,  combined  with  the  reduction  in  stress  resulting  from 
any  increase  in  sag  caused  by  stretching,  explains  the  com- 
parative immunity  from  mechanical  failures. 

Copper. — The  good  qualities  of  copper  wire  are  a  matter  of 
common  knowledge,  and  as  stated  previously,  its  manufacture 
and  method  of  use  combine  to  make  it  almost  unique  as  a  ma- 
terial of  construction.  It  is  fairly  immune  from  corrosive  action, 
as  ordinarily  used  in  transmission-line  work,  although  it  is  not 
absolutely  indestructible.  The  principal  sources  of  injury  to 
copper  are  due  to  its  softness  and  low  melting  point.  The  former 
renders  it  liable  to  nicks  or  broken  strands  in  stringing  and 
clamping,  and  the  latter  to  burning  by  arcs. 

43 


44  POLE  AND  TOWER  LINES 

Unless  the  voltage  is  such  that  insulated  or  weatherproof  wire 
affords  some  real  protection,  there  is  no  logical  structural  reason 
for  using  it.  Otherwise,  it  merely  serves  as  an  additional  load 
and  offers  a  greater  diameter  for  sleet  deposits,  besides  deteriorat- 
ing far  in  advance  of  the  rest  of  the  construction  and  frequently 
hanging  in  unsightly  streamers. 

Since  the  sheen  of  freshly  strung  copper  is  greatly  lessened  after 
exposure,  it  becomes  problematical  whether  the  attention  of  the 
casual  observer  would  in  reality  be  attracted  more  by  the  copper 
or  by  the  size  and  spacing  of  insulators  which  cannot  be  disguised. 

Copper  Covered. — A  comparatively  recent  development  in 
transmission-line  wires  is  the  use  of  a  steel  wire  covered  with 
copper.  This  product  is  produced  by  drawing  out  an  ingot  of 
steel  which  has  been  previously  encased  in  a  copper  covering. 
The  thickness  of  the  shell  of  copper  may  be  varied  within  wide 
limits,  the  usual  commercial 'proportions  being  an  amount  of 
copper  that,  combined  with  the  lower  conductivity  of  the  steel 
core,  produces  a  wire  having  either  30  or  40  per  cent,  of  the 
conductivity  of  a  copper  wire  of  the  combined  gage.  Thirty 
per  cent,  copper-covered  wire  is  about  5  per  cent,  stronger  than 
40  per  cent,  wire  of  the  same  gage.  The  thickness  of  the  shell 
of  copper  is  quite  small,  depending  in  part  on  the  size  of  the  wire. 
Such  wire  should,  therefore,  be  handled  at  least  as  carefully  as 
copper  wire.  Since  the  thickness  of  the  copper  decreases  with 
the  size  of  the  wires  in  the  cable,  it  is  preferable,  at  least  for 
overhead  ground  wires,  to  use  the  40  per  cent,  grade  or  else  to 
use  cables  of  few  strands. 

The  steel  from  which  the  wire  is  drawn  is  a  high-carbon  steel 
having  an  ultimate  strength  of  about  90,000  Ib.  per  square  inch, 
and  a  correspondingly  high  elastic  limit.  During  the  process  of 
copper  coating  and  wire  drawing,  there  is  an  annealing  effect 
followed  by  hardening,  the  net  result  being  to  produce  a  wire 
having  an  ultimate  strength  not  greatly  below  that  of  the  origi- 
nal ingot  material.  In  general,  and  with  the  grade  of  steel  com- 
monly used  by  manufacturing  companies,  the  ultimate  strength 
of  copper-covered  wire  is  from  20  per  cent,  to  40  per  cent,  greater 
than  that  of  the  corresponding  sizes  of  hard-drawn  copper. 

The  principal  uses  of  this  material  in  transmission,  work  are  for 
overhead  ground  wires,  telephone  wires,  and  the  power  wires  of 
the  lighter  and  lower  capacity  lines,  where  little  future  growth  of 
business  may  be  expected. 


WIRES  AND  CABLES 


45 


Aluminum. — Aluminum  wire,  as  now  used  for  power-line  pur- 
poses, is  usually  employed  in  the  form  of  stranded  cables,  and  when 
so  used  is  no  longer  subject  to  some  of  the  troubles  incident  to  the 
earlier  installations.  As  a  material,  it  is  quite  different  from 
copper,  although  used  for  similar  purposes.  Therefore,  in 
making  price  comparisons,  it  is  necessary  to  consider  not  only 
the  price  per  mile  per  unit  of  electrical  rating,  but  also  the  changes 
in  the  general  construction  of  the  line. 

The  conductivity  of  aluminum  is  about  60  per  cent.,  based  on 
the  Matthiesen  standard  for  copper,  making  aluminum  cables 
about  1.5  times  the  area  and  1.25  times  the  diameter  of  copper 
cables  having  equal  conductivity.  As  the  specific  weight  of 
aluminum  is  about  0.33  that  of  copper,  the  weight  of  aluminum 
cable  will  be  about  0.5  times  that  of  copper  cable  having  the 
same  conductivity. 

The  strength  of  aluminum  is  about  0.8  that  of  soft  copper  and 
0.4  the  strength  of  hard  copper. 

The  net  result  of  these  differences  is  best  shown  by  a  concrete 
example.  Thus,  a  No.  00  aluminum  cable  has  about  the  same 
conductivity  as  a  No.  1  stranded  hard-drawn  copper  cable,  and 
their  other  characteristics  for  a  400-ft.  span  are  as  follows: 


Xo.  00 
aluminum 

No.  1 
copper 

Breaking  strength,  pounds  

2500 

3600 

Elastic  limit,  pounds  

1460 

2180 

Wind  pressure  on  ice-covered  diam.,  Ib.  per  foot..  . 
Weight  of  ice-covered  cable,  Ib.  per  foot  
Resultant  load,  Ib.  per  foot.. 

0.943 
0.691 
1  168 

0.885 
0.770 
1  173 

Maximum  wire  tension  (factor  2  0),  pounds 

1250 

1800 

Transverse  load,  per  wire,  pounds  

377 

354 

Normal  sag  

20  ft. 

10ft. 

Maximum  sag.  . 

21  ft. 

12ft. 

Since  the  coefficient  of  expansion  of  aluminum  is  considerably 
higher  than  that  of  copper,  aluminum  cables  are  more  affected 
by  temperature  changes.  Relatively  greater  sags  will  therefore 
occur  in  hot  weather,  while  at  low  temperatures  there  is  a  greater 
increase  in  tension  due  to  the  contraction  of  the  material  with  its 
resultant  decrease  in  sag.  In  addition,  the  lighter  weight  of 
aluminum  renders  it  more  liable  to  local  displacement  by  wind 
pressure.  It  is  necessary,  as  a  result  of  these  differences  in  the 


46  POLE  AND  TOWER  LINES 

material,  to  provide  greater  pin  separation  and  overhead  clearance 
for  aluminum  conductors. 

From  a  construction  viewpoint,  however,  the  lighter  weight  of 
aluminum  does  not  possess  any  particular  merit,  except  possibly 
greater  ease  of  handling  the  reels  and  pulling  out  wire  in  stringing. 
The  saving  in  dead  load  on  the  supports  is  negligible  and  is  more 
than  offset  by  the  greater  sag  and  separation  required.  Further- 
more, the  increased  diameter  imposes  a  greater  wind  load.  The 
choice  between  aluminum  and  copper  will  depend  on  the 
relative  cost  of  the  two  materials  and  their  accompanying  con- 
struction, together  with  the  allowances  which  can  be  made  for 
the  scrap  values  of  the  two  installations. 

Steel. — Steel  cable  can  be  obtained  of  almost  any  desired 
unit  breaking  strength,  the  commercial  grades  ranging  from 
the  low  grade  steel  of  guy  wire,  which  has  an  ultimate  strength 
of  60,000  Ib.  per  square  inch,  to  steels  of  200,000  Ib.  or  more. 

For  transmission-line  purposes  steel  cable  is  used  chiefly  for 
overhead  ground  wires  or  as  power  wires  for  very  long  spans. 
The  occasional  use  of  steel  messengers  for  telephone  or  insulated 
cables  does  not  involve  any  considerable  quantity  of  such  material 
employed  on  typical  transmission  lines. 

Steel  cables  for  line  work  should  always  be  galvanized,  and 
should  be  larger  than  is  actually  required  for  strength. 

The  galvanizing  of  cable  is  by  no  means  as  permanent  a  protec- 
tion as  the  hot-dip,  unwiped  process  applied  to  structural  steel; 
therefore  some  allowance  should  be  made  for  future  corrosion. 

Despite  the  temptation  to  use  small-gage  cables  made  of  the 
higher  grade  steels,  on  account  of  their  greater  strength,  it  is 
generally  preferable  to  adhere  to  medium  grades  such  as  the 
Siemens-Martin.  Guy-strand  steel  cable  is  the  lowest  com- 
mercial grade  and  its  quality  is  relatively  much  lower  than  that 
of  any  of  the  higher  grades.  Large  diameter  cables,  particularly 
of  high-grade  steel,  are  rather  difficult  to  handle  as  they  are  very 
stiff.  It  should  be  noted  that  all  cables  of  great  strength  require 
special  clamping  attachments  for  dead  ending,  the  ordinary 
quota  of  clips  and  clamps  being  inadequate  to  transmit  the 
tension. 

Telephone  Wire. — Supporting  a  telephone  circuit  on  long-span 
transmission-line  structures  introduces  a  difficulty,  in  that  the 
small  wires  which  are  sufficient  for  telephone  service  do  not 
have  the  mechanical  strength  to  carry  safely  in  long  spans. 


WIRES  AND  CABLES  47 

It  is  practicably  impossible  to  string  any  ordinary  telephone 
wires  so  that  they  will  be  reasonably  secure  on  long-span  lines. 
There  have  arisen,  therefore,  two  general  methods  of  procedure; 
one  is  to  use  larger  and  stronger  wires;  and  the  other  to  con- 
template failure  in  the  telephone  circuit  as  a  necessary  evil. 
Solid  steel  wire  No.  6  BWG,  sometimes  called  river  crossing 
wire,  has  an  ultimate  strength  about  equal  to  that  of  No.  00 
stranded  hard-drawn  copper  and  can,  therefore,  be  strung 
equally  well  in  long-span  lines. 

Catenary. — If  two  ends  of  an  imaginary  wire  having  perfect 
flexibility  and  uniformity  of  material  but  no  ductility  were 
supported  at  two  points  in  the  same  horizontal  plane  the  wire 
would  take  the  shape  of  a  curve  known  as  the  catenary.  For  all 
practical  purposes  it  may  be  assumed  that  actual  wires  and  cables 
possess  the  same  characteristics.  The  curve  of  the  wires  between 
the  supports  is  therefore  known,  if  the  span  and  sag  are  known. 
Inasmuch,  however,  as  the  equation  of  the  catenary  is  rather 
complicated,  while  that  of  the  parabola,  which  closely  resembles 
it,  is  simple,  the  latter  is  usually  employed  instead;  thus, 

y*  =  ax 

"a"  being  a  constant  found  by  substituting  the  known  value 
of  x  for  the  point  on  the  curve  at  the  support,  i.e., 

S2 
a  =  j-j  in  which  S  is  the  length  of  the  span  and  d  is  the  sag. 

Assume  a  uniform  load  on  each  lineal  foot  of  the  span,  and 
imagine  half  of  the  span  removed  and  the  wire  held  in  place  by 
the  tension  T  at  the  middle  of  the  span.  Then  considering  the 
moments  about  the  remaining  support  we  have  the  weight  of 
the  half  span  multiplied  by  its  lever  arm  which  is  one-quarter 
of  the  span,  equals  the  balancing  force  T  multiplied  by  its  lever 
arm  d. 

Since  the  total  weight  W  =  the  weight  per  foot,  X  the  half  span 

TI7  WS 

orTF  =  T 
we  have 

fxf-™ 

Therefore 


48  POLE  AND  TOWER  LINES 

and 

55 
'   Sd 

or,  the  tension  in  the  wire  equals  the  weight  per  foot  times  the 
span  squared  divided  by  eight  times  the  sag. 

It  is  necessary,  however,  to  take  into  consideration  the  effect  of 
a  change  in  length  of  the  wire  due  to  temperature  and  loading,  and 
a  simple  arrangement  of  formulae  in  which  this  is  done  is  given 
below. 

The  following  mathematical  treatment  is  not  new,1  but  the 
writer  has  found  the  arrangement  convenient: 

5  =  span,  in  feet. 
d  =  sag,  in  feet. 

W  =  load  per  lineal  foot  in  plane  of  wire. 
A  =  area  of  wire,  in  square  inches. 
E  =  modulus  of  elasticity. 

6  —  coefficient  of  expansion. 

t  =  change  of  temperature,  in  degrees. 
e  =  elongation  or  change  of  length,  within  elastic  limit. 
Lo  =  length,  in  feet,  of   imaginary  wire  (W  =  0)   at  normal 

temperature. 
Loc  =  length,    in    feet,   of    imaginary   wire,   cold    (t°F.    below 

normal  temperature). 

Loh  =  length,    in    feet,   of    imaginary    wire,    hot    (t°F.    above 
normal  temperature). 

Index  to  Subscripts. — 

No  subscript  =  normal  conditions. 

c  =  cold:  £°F.  below  normal  +  dead  load. 
t-  =  cold :  ice  load  +  dead  load. 
ew  =  cold:  wind  load  +  dead  load. 
iw  =  cold:  ice  +  wind  +  dead  load. 
h  =  hot:    t°Y.  above  normal  +  dead  load. 

WM  is  the  resultant  of  the  vertical  dead  +  ice  loads  and  the 
horizontal  wind  load. 

Wcw  is  the  resultant  of  the  vertical  dead  load  and  the  hori- 
zontal wind  load. 

1  Overhead  Construction  for  High-tension  Electric  Traction  or  Trans- 
mission, by  R.  D.  Coombs,  Trans.  Am.  Soc.  C.  E.,  Vol.  LX  (1908). 


WIRES  AND  CABLES  49 

Stresses.  —  Substitute  normal  values  in  Eqs.  1,  2,  3,  and  4. 
Assume  values  of  Th,  Tiw,  Tc,  Tiy  or  Tew,  such  that  Eqs.  5  and  6 
will  give  identical  values  of  dh,  diw,  etc.  The  tension  that  will 
give  the  same  sag  by  Eqs.  5  and  6  (independently)  is  the  tension 
resulting  from  that  sag  and  the  given  loading. 

'  (2) 


Lo  =  L  -  e  (4) 

(Z°F.  above  normal,  with  dead  load.) 

Loh  =  L0(l  -f  Bth)     eh  =     °  Lh  =  Loh  +  eh 


dh  =  0.612\/,S~(ZA  -  S)  (5) 

,        Wh  XS* 

dh=  ~WT 

(t°~F.  below  normal,  with  dead  +  ice  +  wind  loads.) 

LOC    =  L0(\    —    Otc)          €iw    =  ^Tj  Liw    ==  Loc  ~\~  €iw 

diw  =  O.Ql2Vs(Liw  -  S)  (5) 

,         Wiw  X  S2 
diw=    ~ST~~ 

(t°F.  below  normal,  with  dead  load.) 

L    X  T 

Loc  =  L0(l  —  dtc)       ec  =     °Cg£  Lc  =  LOC  +  ec 

dc  =  0.612V/^(LC  -  S)  (5) 

Wc  X  S* 

dc  =  ~wr~  ('6) 

(t°F.  below  normal,  with  dead  +  ice  loads.) 

Lv  T. 

Li      i  -i  r\i    \  oc    ^          *  T  T  I 

<,c  =  La(l  —  ete) 

di 
d 

di 


i  =  0.612s(Li  -  S)  (5) 

Wi  X  S' 


50 


POLE  AND  TOWER  LINES 


(t°F.  below  normal,  with  dead  +  wind  loads.) 

X   J-  cw 


Loc  =  L0(l  —etc)       ecw  = 


EA 


—  L 


or     I     c/cto 


dcw  =  0,612V  S(LCU,  -  S) 
Wcw  X  S2 


dr.,:,     = 


8Te 


(5) 
(6) 


In  Tables  6  to  13  are  given  the  physical  properties  and  the  wind 
and  ice  loads  for  various  wire  gages.* 

TABLE  6. — PROPERTIES  OF  WIRE  MATERIAL 


Ultimate 
strength,  Ib.  per 
square  inch 

Elastic  limit, 
Ib.  per  square 
inch1 

Modulus  of 
elasticity, 
E 

Coefficient 
of  expan- 
sion, per  °F. 

Copper,  solid  soft-drawn  
Copper,  solid  med.-drawn  

32  to    34,000 
40  to    50,000 

16,000 
22  to  27,000 

14,000,000 
15,000,000 

0.0000096 
0  .  0000096 

Copper,  solid  hard-drawn  

50  to    60,000 

30  to  35,000 

16,000,000 

0  .  0000096 

Copper,  strand  soft-drawn  
Copper,  strand  med.-drawn  
Copper,  strand  hard-drawn  
Copper  clad,  solid,  hard-drawn..  . 
Copper  clad,  strand  hard-drawn.  . 
Aluminum,  strand  

30,000 
45,000 
55,000 
60  to    90,000 
70  to    90,000 
23  to    24,000 
75  000 

15,000 
25,000 
33,000 
35  to  53,000 
41  to  53,000 
14,000 

8,000,000 
10,000,000 
12,000,000 
21,000,000 
18,000,000 
9,000,000 
25  000  000 

0.0000096 
0.0000096 
0  .  0000096 
0  .  0000067 
0.0000067 
0.0000128 
0  0000066 

150  000 

25  000  000 

0  0000066 

180  000 

25  000  000 

0  0000066 

Steel  solid  ex-high-strength 

187,000 

29,000  000 

0  0000066 

It  has  been  urged  by  some  that  using  the  0.5-in.  ice  and  8-lb. 
wind  load  with  the  parabolic  formula  for  computing  the  stress 
in  the  wire  does  not  give  results  which  accord  with  experience, 
since  actual  spans  erected  with  less  than  the  specified  sags  have 
not  failed  in  service.  On  the  other  hand,  it  is  sometimes  claimed 
that  allowing  maximum  stresses  near  the  elastic  limit  is  danger- 
ous. The  facts  of  the  matter  are: 

First,  the  true  catenary  formula  is  scientifically  and  mathe- 
matically correct  within  the  elastic  limit.  Second,  the  para- 
bolic formula,  ordinarily  used  for  simplicity,  gives  results  which 
in  the  vast  majority  of  cases  are  closer  to  the  exact  values  than 
the  actual  wire  stringing  will  be  to  the  specified  stringing.  Third, 
the  material  of  a  wire  catenary  is  more  uniform  in  section, 

*  Tables  from  R.  D.  Coombs  &  Co.  design  standards. 

1  The  elastic  limit  used  is  in  reality  the  yield  point,  or  point  of  appreciable 
extension,  as  this  value  seems  more  applicable  to  wire  stringing  than  that 
obtained  by  accurate  laboratory  tests. 


WIRES  AND  CABLES 


51 


strength,  and  other  characteristics  than  that  of  any  other  engi- 
neering structure;  therefore  the  error  of  design  is  correspondingly 
less.  Fourth,  the  stretch  of  a  ductile  material,  such  as  copper, 
permits  the  sag  to  increase  and  the  stress  to  decrease  and,  within 
limits,  does  not  perceptibly  decrease  the  cross-section  at  any 


1 

42 

Span  in  Feet 
00           200           300            400           500           600            7C 

K)          800          900 

I 

/ 

7 

I 

42 
40 
38 

I 

7 

38 
3C 
34 
32 
30 
28 
26 
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18 

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30 
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30           200            300          400           500           600           700           800          900 
Span  in  Feet 

FIG.  24. — Sags  and  tensions  of  No.  1  H.  D.  copper. 


point.  Therefore,  a  loaded  span  stretches  enough  to  relieve  the 
stress  and  does  not  fail  unless  the  load  is  very  excessive. 

Fifth,  the  specified  maximum  loading  is  an  emergency  loading 
and  is  not  a  general  or  frequent  occurrence  on  any  span  or  line. 

The    facts   of   the    matter    are   that   the    spans    in   service 


52 


POLE  AND  TOWER  LINES 


have  either  not  been  subjected  to  loads  in  excess  of  those  re- 
quired by  the  catenary  formula  to  develop  their  elastic  limit, 
or  the  wire  has  stretched  and  the  sag  has  increased.  Pos- 
sibly there  are  two  other  reasons  for  seemingly  overstressed  lines 
giving  satisfactory  service.  One  is  that  the  poles  have  bent 


100 


34 


200 


Span  in  Feet 
400  500 


700 


800 


"v 


A 


/y 


900 

•—144 


100     200     300     400     500     600     700     800     900 
Span  in  Feet 

FIG.  25. — Sags  and  tensions  of  No.  0  H.  D.  copper. 


or  the  wires  slipped  through  the  ties,  thus  temporarily  increasing 
the  sag.  Second,  the  wires  may  have  become  tempered  or 
hardened  by  tension,  possibly  by  atmospheric  changes  or  other 
action,  and  by  becoming  harder  have  been  able  to  sustain  a 
greater  load. 


WIRES  AND  CABLES 


53 


100  200  300 


c22 

18 
16 
14 
12 


Span  in  Feet 
400  500  600  700  800          900 


*/ 


-v/ 


Lit 


100  200  300 

FIG.  26. — Sags 


400  500  600  700  800 

Span  in  Feet 

and  tensions  of  No.  00  H.  D.  copper. 


54 


POLE  AND  TOWER  LINES 


188! 


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POLE  AND  TOWER  LINES 


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58 


POLE  AND  TOWER  LINES 


1 

C  7 


I      II      I 


Copper  Wire 

Normal  Sags,  (60  F.  no  Ice  or  Wind) 
Factor  of  Safety,  at  Max  Load =2.0 
Max.  Load=^'lce+8.0  Lb.  Wind  0°F. 


Span  in  Feet 
FIG.  27. — Normal  sags,1  copper  wires  and  cables. 


'Overhead  Line  Construction  Committee  (N.E.L.A.). 


WIRES  AND  CABLES 


59 


. 

\. 

00 

*000 
0000 

Stranded  Aluminum  Wi 
Normal  Sags,(60°F.  no  Ice  or 
Factor  of  Safety,  at  Mar.  Loa 
Max.  Load-J^'lce+8.0  Lb.  Wl 

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FIG.  28. — Normal  sags,1  aluminum  cable 


TABLE  13. — PROPERTIES  OF  WIRE  MATERIAL 

(From  1911  Report  of  Overhead  Line  Construction  Committee  of  National  Electric  Light 

Association) 


Ultimate 
strength 
per  sq.  in. 

Elastic  limit 

Modulus 
elasticity, 
E 

Coefficient 
of 
expansion 

Copper,  solid,  soft-drawn  

32-34,000 

28,000 

12,000,000 

0.0000096 

Copper,  solid,  hard-drawn  

50-55-57-60,000 

30-32-34-35,000 

16,000,000 

0.0000096 

Copper,  stranded,  soft-drawn  .  . 

34,000 

28,000 

12,000,000 

0.0000096 

Copper,  stranded,  hard-drawn. 

60,000 

35,000 

16,000,000 

0.0000096 

Aluminum,  stranded  

23-24,000 

14,000 

9,000,000 

0.0000128 

Steel,  stranded,  Siemens-Martin 

75,000 

29,000,000 

0  .  0000064 

Steel,  stranded,  high-tension.  .  . 

125,000 

29,000,000 

0.0000064 

Steel,  stranded,  ex-high-tension 

187,000 

29,000,000 

0  .  0000064 

Overhead  Line  Construction  Committee  (N.E.L.A.) 


CHAPTER  IV 


DESIGN 

Since  it  is  impracticable  to  include  herein  a  sufficient  explana- 
tion of  the  laws  of  mechanics  or  the  theory  and  practice  of 
structural  design,  to  enable  the  inexperienced  to  acquire  even  a 
reasonable  facility  in  their  use,  no  detailed  exposition  thereof 
has  been  attempted.  The  computation  of  stresses,  and  the 
determination  of  sections  for  such  structures  as  steel  towers, 
requires  a  working  knowledge  of  subjects  already  covered  by 
various  text-books.  However,  there  are  a  number  of  general 
conditions  in  which  transmission-line  work  does  not  follow  the 


'V*  /frfe~ 


FIG.  29. — Relative  strength  of  telephone  and  power  lines. 

accepted  standards  and  methods  of  other  structural  design,  and 
a  discussion  of  such  matters  should  be  of  value  to  otherwise 
competent  designers  whose  experience  has  been  obtained  in  a 
different  field. 

Factors  of  Safety,  Etc. — If  a  given  line  is  to  be  designed  in  a 
logical  manner  and  with  a  minimum  of  "cut  and  try"  methods, 
the  first  step  is  the  assumption  of  the  various  loads  and  factors 
of  safety.  These  assumptions  will  enable  the  designer  to  men- 

60 


DESIGN  61 

tally  predetermine,  to  some  extent,  the  general  nature  of  the 
supports,  or  at  least  to  narrow  the  field  of  choice.  In  a  broader 
sense,  it  will  also  limit  the  choice  to  one  kind  of  material  for  the 
supports,  since  a  wooden-pole  line  cannot  have  a  total  factor  of 
safety  equal  to  that  possible  in  steel  construction. 

The  first  and  easiest  factor  of  safety  to  assume  is  that  for  the 
wires.  It  is  the  easiest  to  assume,  since  once  chosen  it  can  be 
maintained  without  much  effect  upon  the  type  of  support.  Again, 
a  reasonable  factor  in  the  wires  will  have  only  a  beneficient  effect 
upon  all  the  remaining  construction.  Further,  there  is  a  more 
general  consensus  of  opinion  regarding  this  assumption  than  on 
any  other  element  affecting  transmission-line  construction.  The 
literal  expression  of  various  wire  loads  and  factors  may  appear 
quite  dissimilar,  but  the  ultimate  result  when  the  wires  are  strung 
presents  a  fair  average.  Unbalanced  loads  and  factors  of  safety 
have  been  very  common,  and  they  are  greatly  to  be  condemned 
both  as  a  misstatement  of  fact  and  as  providing  an  excuse  for 
future  errors  of  design. 

Sleet  may  be  encountered,  during  the  probable  life  of  a  well-built 
transmission  line,  in  a  great  many  sections  throughout  America, 
even  in  the  South.  Moreover,  it  may  be  desirable  to  provide 
a  sag  corresponding  to  the  standard  sleet  load,  even  in  non-sleet 
regions,  because  such  sags  decrease  both  the  normal  and  the 
maximum  loads  on  the  line  and,  in  general,  produce  a  line  which 
is  well  able  to  distribute  and  equalize  excess  stresses. 

In  general  the  "J^-in.  ice  plus  8.0-lb.  wind"  loading  is  logical 
in  origin,  as  shown  by  the  writer  in  1908;1  further  it  is  more 
universally  accepted  than  any  other. 

A  factor  of  safety  in  wires  of  2.0  is  approximately  equivalent 
to  a  working  stress  of  0.9  of  the  elastic  limit,  and  so  is  not  too 
conservative  when  errors  of  stringing  and  possible  reduction  of 
strength  at  splices  are  considered. 

An  exception  may  be  made  to  the  above  loads  and  factor  in 
the  case  of  very  long  spans,  in  which  the  load  may  be  consistently 
reduced  about  25  per  cent.  Thus  in  designing  one  line  with 
800-ft.  spans,  the  writer  believes  that  his  use  of  a  6.0-lb.  wind 
pressure  was  logical.  An  additional  exception  to  the  above 
factor,  if  not  to  the  load,  is  the  short-span  distribution  lines 
in  city  streets.  Such  lines  are  usually  well  sheltered,  designed 
for  low  voltages,  heavy  and  numerous  wires,  and  better  guyed 

1  Proceedings  American  Society  of  Civil  Engineers,  1908. 


62  POLE  AND  TOWER  LINES 

when  the  lateral  restraint  of  wires  and  the  guys  at  street  corners 
are  considered.  As  proven  by  actual  experience  with  many 
thousands  of  miles  of  such  lines,  they  may  consistently  be  given 
a  factor  of  one  (1)  with  the  above  maximum  loading. 

It  is  true  that  many  transmission  lines  have  been  built  with 
tighter  stringing  than  recommended  above — only  a  few  having 
had  more  conservative  values — but  as  a  general  statement,  slack 
lines  are  safer.  If  the  number  of  wires  will  permit  liberal  separa- 
tion of  the  conductors,  a  little  increase  in  the  sag  will  often 
help  out  in  the  design  of  the  supports.  It  seems  probable  that 
for  every  span  in  which  slack  stringing  (and  improper  separation) 
have  caused  accidental  contacts  there  have  been  three  cases  in 
which  tight  stringing  has  broken  pins,  insulators  or  supports. 

After  making  the  foregoing  assumptions  there  still  remain  to  be 
determined  certain  loads  and  factors  for  the  supporting  structures. 
The  specified  wire  load  applies  also  to  the  supports,  but  in  addition 
there  is  the  broken- wire  load  to  be  considered,  as  well  as  the  factor, 
or  factors,  for  the  poles  or  towers.  Broken-wire  loads  are  dis- 
cussed in  more  detail  elsewhere  (pages  39  to  42),  the  writer's 
recommendation  being  that  for  transmission  lines  the  effect  of 
one  broken  wire  should  be  combined  with  the  above-mentioned 
ice  and  wind  loads,  while  for  short-span  city  distribution  lines 
broken-wire  loads  may  be  neglected. 

While  the  requirements  just  mentioned  would  literally  apply 
to  insulators  and  pins  on  all  supports,  it  is  impracticable  and  un- 
necessary to  so  interpret  the  broken- wire  load.  It  will,  there- 
fore, apply  only  to  insulators  and  pins  at  corners,  dead  ends, 
crossings  and  special  points.  On  intermediate  tangent  poles 
single-pin  insulators  and  tie  wires  may  properly  be  used,  although 
they  may  not  always  have  the  required  broken-wire  strength, 
and  the  broken-wire  effect  on  the  pole  might  be  considered 
as  the  equivalent  result  of  one  broken  wire  or  of  several  un- 
balanced wires. 

Owing  perhaps  to  unfamiliarity  with  the  structural  questions 
involved  in  the  construction  of  transmission  lines,  engineers  have, 
until  recently,  specified  the  test  loads  which  sample  towers  or 
poles  must  withstand.  Dependence  on  a  large,  and  more  or  less 
certain,  factor  of  safety  to  cover  uncertain  design,  however, 
should  have  no  permanent  place  in  line  construction.  On  the 
other  hand,  the  theory  of  this  practice,  i.e.,  that  of  working  to 
known  ultimate  strengths,  has  much  to  commend  it.  Moreover, 


DESIGN  63 

tests  of  sample  towers  have  been  of  considerable  use  in  adding  to 
our  somewhat  meager  store  of  data  on  the  ultimate  strength  of 
columns. 

Unfortunately  for  the  entire  success  of  this  procedure,  a 
test  load  is  very  rarely  an  accurate  representation  of  the  maximum 
which  may  be  obtained  in  practice,  nor  is  the  condition  of  the  test 
structure  similar  to  that  of  many  of  the  structures  when  installed. 
Test  loads  are  almost  always  applied  regularly  and  slowly,  and 
in  many  cases  uneccentrically.  A  test  structure  most  assuredly 
will  have  at  least  a  fairly  good  foundation,  and  be  composed  of 
members  free  from  incipient  bends  or  other  defects  caused  by 
mishandling.  It  would  also  be  very  well  bolted  together  and 
plumbed  with  greater  accuracy  than  the  average  line  structure. 
In  general  it  may  be  said  that  an  expert  structural  assembler 
should  be  able  to  obtain  test  loads  quite  noticeably  in  excess  of  the 
presumptive  average  strength  of  the  finally  erected  structures. 
It  appears,  therefore,  that  the  period  of  usefulness  for  this 
practice  is  past,  and  that  competent  designers  should  be  able  to 
produce  structures  which  will  have  actual  strengths  much  nearer 
their  predetermined  strengths  than  the  actual  loads  will  be  to  the 
assumed  loads.  If  statically  indeterminate  frames  are  used,  such 
as  poles  with  incomplete  web  systems,  design  tests  are  necessary, 
but  the  transfer  of  stresses  and  the  construction  of  efficient  details 
are  now  so  well  understood  that  actual  tests  of  determinate 
structures  are  in  a  measure  a  confession  of  ignorance. 

The  failure  of  steel  poles  and  towers  has  almost  invariably  been 
caused  by  the  buckling  of  main  compression  members,  and  this 
may  or  may  not  be  superinduced  by  inefficient  bracing.  Owing 
to  the  possible  application  of  the  load  from  the  opposite  side  of  a 
structure,  line  supports  must  have  the  same  main  compression 
section  at  each  corner  regardless  of  the  tension  stress.  The  com- 
pression stress  per  square  inch  in  the  main  legs  is,  therefore,  the 
first  and  most  important  determination.  A  secondary  condi- 
tion which  should  be  borne  in  mind  during  the  calculations  is 
that  the  section  selected  must  be  of  a  size  suitable  for  the  connec- 
tion of  the  desired  bracing. 

Admitting  that  there  is  a  difference  in  the  kind  of  service  or, 
in  all  events,  in  the  number  of  applications  of  the  load,  between 
transmission  and  building  work,  there  is  a  very  marked  variation 
in  the  attitude  of  engineers  toward  bolted  connections  in  the  two 
types  of  construction.  In  building  work  single  bolts  are  dis- 


64  POLE  AND  TOWER  LINES 

couraged  or,  if  used,  low  strength  values  are  allowed.  In  towers, 
however,  all  joints  are  bolted,  and  then  usually  with  one-bolt 
connections  in  which  no  reduction  of  value  is  assumed. 

The  theoretical  value  of  a  one-flange  connection  should  be 
reduced  as  the  full  strength  of  the  connected  member  is  not 
available ;  this  has  not  been  so  assumed  in  tower  work.  Again,  the 
bearing  of  a  bolt  is  assumed  as  being  on  a  surface  as  thick  as 
the  member;  in  fact  the  bearing  will  be  only  on  a  line. 

The  strength  of  a  one-flange  connection  is  approximately  but 
80  per  cent,  of  the  strength  of  the  angle;  therefore,  since  many 
tower  joints  are  one-flange  connections,  a  somewhat  conservative 
unit  stress  should  be  assumed  in  the  design  of  bracing.  Further, 
if  a  member  has  one  flange  "blocked  off,"  i.e.,  cut  entirely  away 
for  clearance,  or  if  the  flanges  are  mashed  together  or  flattened 
out,  the  strength  of  the  member  at  that  point  is  no  longer  the 
strength  of  an  angle  but  of  a  flat.  In  addition,  there  is  con- 
siderable likelihood  that  such  blacksmith  work  may  result  in 
burning  or  cracking  the  material  at  the  point  in  question. 

As  a  long  slender  member  is  not  well  adapted  to  withstand 
compression,  it  has  been  customary  in  other  work  to  limit  the 
relation  of  the  length  to  the  radius  of  gyration.  In  transmission- 
line  construction,  however,  very  much  higher  values  of  this  ratio 
have  been  used  than  are  generally  permitted.  It  is  probably  not 
necessary  to  adhere  to  the  low  limits  of  building  construction, 
but  it  is  equally  probable  that  too  much  latitude  has  been 
taken  in  some  cases  heretofore. 

The  more  recent  designs  of  transmission  line  supports  do  not 
employ  any  castings  in  the  main  structure,  although  cast-steel  or 
malleable-iron  castings  are  perhaps  properly  applicable  to  wire 
connections.  Similar  reasoning  should  prohibit  the  use  of 
castings  for  hoops  or  bands  in  reinforced-concrete  poles. 

Transverse  Loads. — Before  entering  upon  any  detailed  dis- 
cussion of  design,  it  is  necessary  to  consider  briefly  the  forces 
acting  upon  a  pole  line  and  the  character  of  service  required  of 
its  component  parts.  As  already  stated,  the  function  of  the  pole 
is  that  of  a  cantilever  beam  rather  than  of  a  column.  The 
external  forces  are  due  to  dead,  ice,  and  wind  loads,  which,  with 
the  exception  of  the  pressure  on  the  pole,  must  be  transmitted  to 
the  pole  by  the  wires. 

The  weight  of  the  wires  and  their  coating  of  sleet,  together 
with  the  weight  of  crossarms,  insulators,  and  the  pole  itself, 


DESIGN 


65 


is  a  vertical  load  which  the  pole  carries  as  a  column.  The 
pressure  of  the  wind  on  the  wires,  whose  diameter  is  in- 
creased by  the  sleet,  and  upon  the  pole  structure,  is  assumed 
as  acting  horizontally  and  at  a  right. angle  with  the  line,  and, 


1—4- 


E -s  — 


--Si- 


Pole  A 
FIG.  30.  —  Transverse  loading. 


Pole  A 


Pi  =  Transverse  load  at  ground  wire  =  wind  load  per  ft.  of  wire  a(~^-\ — n  )• 
Pa  =  Transverse  load  at  top  power  wire  =  wind  load  per  ft.  of  wire  b(-  -\ — -^  . 

Pt  =  Transverse  load  at  lower  power  wires  =  wind  load  per  ft.  of  wire  &(o +~<r)  x2- 
P4  =  Wind  load  per  ft.  of  pole  X  length  of  pole  above  ground. 
V  =•  Vertical  load  on  pole  = 

+  weight  of  pole. 


weight  per  lin.  ft.  of  wire  X  no.  of  wires  X    ~  H  —  ~ 


therefore,  its  effect  is  much  greater  than  the  effect  of  the  vertical 
forces. 

When  poles  are  closely  spaced  there  is  undoubtedly  some  side- 
guying  effect  due  to  lateral  restraint  from  the  wires.  That  is,  if 
one  pole  is  subjected  to  a  severe  gust  of  wind,  the  neighboring 
poles  will  be  brought  into  action,  to  a  limited  extent,  by  the 
wires  spreading  out  some  of  the  load  to  adjacent  poles. 

Corner  Loads.  —  In  the  case  of  a  pole  placed  at  a  bend  in  the 
line,  there  must  be  added  to  the  foregoing  the  transverse  com- 
ponent of  the  tension  in  the  wires,  i.e.,  the  maximum  tension 
multiplied  by  twice  the  sine  of  one-half  the  angle  of  the  bend. 


Let  A  O  B  be  a  Line  of  Wires 
making  the  Angle  A  at  O 


FIG.  31. — Corner  loading. 


POLE  AND  TOWER  LINES 


A  little  study  of  Fig.  32  will  explain  why  corners  are  usually 
the  weakest  points  in  a  line,  since  they  are  unquestionably  and 
invariably  the  only  supports  subject  to  continual  loading  and 
because  a  portion  of  the  load  alone  may  be  considerably  more  than 
a  dead-ending  load.  In  the  writer's  opinion,  considerably  more 
effort  and  money  than  has  usually  been  expended  is  amply 
warranted  in  avoiding  or  decreasing  curves  and  corners. 


Transverse  Load  on  Pole 
(In  Terms  of  Wire  Tension) 
p  p  p  f>  p  f-  £  j-  £  H-  jo 

^ 

^ 

-^•—  "" 

X 

y 

X 

^ 

^ 

/ 

/ 

/ 

/ 

A 

/ 

/ 

/ 

/ 

' 

/ 

/ 

/ 

/ 

/ 

/ 

/ 

/ 

/ 

0°    10°  20°  30°   40°  50°   60°  70°   80°  90°  100°110°1200 130°  140°150°  160°  170°180° 

Degrees 

FIG.  32. — Corner  pole  loading.1 

Following  the  common  practice  in  low-voltage  construction,  the 
statement  is  frequently  made  that  at  corners  the  neighboring 
spans  should  be  shortened  to  minimize  the  stress  on  the  corner 
structure. 

In  the  absence  of  further  elaboration,  and  in  view  of  the 
usual  lack  of  expert  advice  during  erection,  it  would  appear 
that  the  above  statement  is  thought  to  be  self-sufficient. 

1  R.  D.  Coombs  &  Co.  Design  Standards. 


DESIGN  67 

This  is  decidedly  not  the  case,  since  the  stress  to  which  the  corner 
support  is  subjected  is  not  reduced  materially  by  shorter  spans, 
unless  advantage  is  taken  of  the  short  spans  to  increase  the  sag 
in  those  spans.  This  is  due  to  the  fact  that  the  greater  part  of 
the  load  upon  the  corner  support  is  from  the  tension  in  the  wires, 
and  unless  this  tension  is  reduced  by  slack  stringing,  there  is  no 
particular  advantage  in  short  spans.  x 

With  short-span  low- voltage  construction,  an  increase  in  sag 
of  a  few  inches,  made  by  the  line  foreman  to  "ease  up  the  corner," 
will  be  inconspicuous  and  very  efficient,  provided  that  the  tying-in 
of  the  wires  is  effective  for  some  distance  each  side  of  the  corner. 
Long-span  high- voltage  construction,  however,  requires  a  material 
change  in  sag  and  there  is  a  redistribution  of  stresses  to  be  pro- 
vided for,  unless  entire  dependence  is  to  be  placed  on  the  bending 
of  supports  and  on  the  slipping  of  wires  at  the  supports. 

Slack  corner  spans  may  have  a  maximum  wire  tension  from 
500  to  1000  Ib.  less  than  the  standard  stringing;  so  if  this  reduc- 
tion is  to  remain  effective  the  unbalanced  tension  must  either 
be  held  by  and  at  the  adjoining  supports  or  be  carried  back  and 
distributed  over  a  number  of  supports. 

It  is  needless  to  say  that  the  standard  stringing  curve,  in  case 
one  is  provided,  is  not  applicable  to  such  construction.  Further, 
it  is  useless  to  attempt  to  distribute  unbalanced  wire  tension  by 
means  of  a  slip-shod  single  pigtail  tie  wire. 

Broken-wire  Loads. — In  case  the  sags  in  adjoining  spans  are 
not  adjusted  so  as  to  balance  the  tension  in  the  wires  each  side 
of  a  pole,  there  will  be  an  unbalanced  pull  in  the  direction  of  the 
line,  which  must  be  considered  in  conjunction  with  the  vertical 
and  horizontal  forces  first  mentioned.  Unbalanced  tension  may 
also  be  produced  by  unequal  ice  and  wind  loads  in  adjoining 
spans.  Further,  if  it  is  assumed  that  all,  or  part,  of  the  wires 
may  be  broken,  then  the  poles  must  withstand  a  longitudinal 
force  equal  to  the  tension  in  the  wires  in  the  unbroken  span. 

On  the  other  hand,  it  can  be  shown  by  a  rather  complicated 
mathematical  demonstration  that,  owing  to  certain  properties 
of  the  catenary  curve,  a  slight  bending  in  a  number  of  poles  will 
balance  the  tensions  in  adjoining  spans.  This  is  due  to  the  fact 
that  the  tension  in  a  wire  is  greatly  decreased  if  the  span  length 
is  shortened  while  the  length  of  wire  per  span  remains  unchanged. 
Vice  versa,  increasing  the  span  length  while  the  length  of  wire 
per  span  remains  unchanged  increases  the  tension. 


68  POLE  AND  TOWER  LINES 

If  it  is  assumed  that  all  the  wires  in  one  span  are  broken,  then 
the  first  pole  is  subject  to  the  unbalanced  tension  of  all  the  wires 
in  the  unbroken  span  and  bends  away  from  the  break.  This 
shortens  the  next  span  length,  decreases  the  tension  in  that  span, 
and  allows  the  second  pole  to  be  bent  away  from  the  break. 
Successive  bending  occurs  in  decreasing  amounts,  until  a  point 
of  equilibrium  is  reached  at  which  the  wire  tension  next  to  the 
break  is  considerably  less  than  the  original  tension. 

If  it  is  assumed  that  less  than  the  entire  number  of  wires  are 
broken,  then  the  bending  of  the  first  pole  increases  the  span  length 
of  the  remaining  wires  and  by  increasing  their  tension  causes  them 
to  exert  a  greater  pull  toward  the  break  and  thus  decreases  the 
unbalanced  pull  on  the  pole. 

However,  the  ordinary  attachments  for  fastening  line  wires 
to  the  insulators  do  not  always  have  sufficient  strength  to  develop 
the  strength  of  the  wire  and,  therefore>  a  broken  wire  would  pull 
through  into  the  adjoining  spans  before  exerting  its  maximum  ten- 
sion on  the  poles.  For  this  reason,  and  because  equilibrium  by 
bending  may  result  in  over-stressing  the  poles,  wires,  pins  or 
insulators,  it  is  not  always  possible  to  take  advantage  of  this 
method  of  design. 

In  assuming  the  possibility  of  broken  wires,  it  becomes  neces- 
sary to  assume  which  wires  may  break  as  well  as  their  number. 
If  the  wires  farthest  from  the  pole  are  broken,  the  effect  on  the 
crossarms  is  much  greater  than  in  the  case  of  wires  near  the  pole. 
If  all  the  broken  wires  are  on  one  side  of  the  pole,  the  torsional 
effect  on  the  pole  must  be  considered. 

Column  Formulas. — Inasmuch  as  the  strength  of  the  main- 
leg  members  of  a  pole  or  tower,  as  well  as  most  of  the  bracing, 
is  predicated  upon  their  strength  as  compression  members, 
the  most  important  requirement  of  a  specification  next  to  the 
broken-wire  condition  is  the  formula  for  compression  members, 
known  as  the  column  formula.  Unfortunately,  the  many  column 
formulas  in  existence  are  always  expressed  in  terms  of  "safe 
working  unit  stresses,"  which  renders  them  almost  valueless  to 
the  inexpert  transmission-line  designer,  unless  their  factor'  of 
safety  is  known.  This  is  due  to  the  fact  that  in  transmission 
line  construction  it  is  the  ultimate  or  breaking  strength  which 
must  be  determined  in  order  that  a  specified  factor  of  safety 
may  be  applied  thereto. 

The  writer  is  aware  that  the  last  statement  may  be  criticised 


DESIGN  69 

as  a  defense  of  a  dangerous  practice,  in  that  no  protection  is 
afforded  by  the  working  stress  for  incompetence  on  the  part  of 
the  designer.  On  this  point  the  writer  wishes  to  emphasize  his 
belief  that,  provided  a  portion  of  the  factor  of  safety  is  present 
to  offset  minor  errors  of  design,  the  unit  stress  should  not  be  ex- 
pected to  afford  such  protection.  As  a  matter  of  fact,  when  bid- 
ding under  a  specification  requiring  a  test  in  which  a  sample  pole 
or  tower  "must  withstand"  certain  loads,  the  competing  de- 
signers are  compelled  to  work  as  close  to  the  probable  ultimate 
or  buckling  strength  as  seems  to  them  advisable.  Therefore,  it 
would  serve  to  eliminate  the  personal  equation  of  the  designer, 
together  with  the  false  security  arising  from  test  towers,  if  there 
were  available  accurate  rules  by  which  to  compute  the  ultimate 
strength  of  statically  determinate  structures. 

In  general  there  are  such  rules,  and  aside  from  the  difficulties 
or  inaccuracies  of  computing  eccentric  or  torsional  stresses,  the 
chief  uncertainty  is  in  the  column  formula.  The  engineering 
profession  has  long  awaited  a  complete  series  of  ultimate  com- 
pressive  tests  and  the  derivation  therefrom  of  a  set  of  generally 
accepted  column  formulas. 

It  is  to  be  regretted  that  the  hundreds  of  tower  tests  which 
have  been  made  to  date  have  not  resulted  in  a  more  accurate 
and  more  general  addition  to  our  knowledge  of  the  subject. 

In  pole  and  tower  design,  the  compression  members  are  simple 
in  type,  usually  single  angles  with  relatively  large  ratios  of  the 

unsupported  length  to  the  radius  of  gyration,  i.e.,  —     "Failure" 

occurs  when  such  members  buckle,  as  the  structure  becomes  dis- 
torted and  useless,  although  it  may  not  fall  to  the  ground.  It  is 
readily  apparent  that  any  incipient  bends  in  such  columns  will 
very  markedly  affect  the  theoretical  compressive  strength.  In 
addition  it  is  quite  possible  to  select  sections  such  as  4  in.  X  4 
in.  X  J-i-in.  angles,  for  example,  whose  theoretical  strength  by 
the  column  formula  exceeds  their  actual  strength.  This  is  due 
to  the  fact  that  in  such  large  thin  sections  failure  may  start  by 
the  local  buckling  of  the  legs  of  the  angle. 

Columns  have  been  divided  into  classes  according  to  the  nature 
of  their  end  connections,  whether  "fixed,"  "pin-ended,"  or  "round- 
ended"  and  free  to  move.  A  pole  set  in  an  adequate  concrete 
foundation  probably  approaches  the  condition  of  one  fixed  and 
one  free  end,  if  the  whole  pole  is  under  consideration.  The 


70 


POLE  AND  TOWER  LINES 


columns  formed  by  tower  members  in  general  might  be  con- 
sidered as  stronger  than  flat-ended  and  weaker  than  fixed-ended 
columns. 

A  discussion  or  compilation  of  the  various  column  formulas 
and  their  derivation  is  beyond  the  scope  of  this  book,  but  in  Fig. 
31  are  shown  several  formulas  expressed  in  terms  of  ultimate 


42000 

4 

40000 

\ 
\ 

OOAAA 

^> 

\ 

T" 

,3 

2 

\ 

V 
\ 

\ 

\ 
\ 

Y 

\\ 

\ 

\ 

,  V 
\     \ 

\ 

N1 

\ 
\\ 

\ 
\ 

\ 

s\ 

\ 

28000 

V 

% 

\ 
\ 

\ 

v 

s 

\ 

to  ~  24000 

\v 

\ 
\ 

M  «  2200° 

N 
4^ 

s\ 

\ 

^ 

-S  *3  1ROOQ 

•*-^ 

X 

\ 

5 

IfiOOO 

\ 

\ 

\ 
\ 

\ 

\ 

\ 

^ 

2 

\ 

^ 

\  \ 

\ 

\ 

^ 

2s  o 

\ 

\ 

8000 

x 

o 

^ 

\ 

3 

\ 

x 

6000 

\ 

^v. 

^x 

>^ 

1 

4000 

""-- 

^. 

•—  -. 

2000 

n 

0      20     40 

Ratio   of  — 

FIG.  33. — Column  formulas. 

strength.  It  will  be  noted  that  the  chief  differences  are  at  the 
ends  of  the  curves,  i.e.,  either  for  very  small  or  very  large  values 
of  l/r.  It  should  be  noted  that  the  ultimate-strength  curves  are  based 
on  the  assumption  of  good  design,  workmanship  and  material,  the 
use  of  medium  steel,  and  freedom  from  incipient  injury.  They 


DESIGN 


71 


represent  approximate  average  values  and  must  be  used  with  caution 
by  the  inexpert.  They  are  chiefly  useful  in  showing  the  derivation 
of  the  allowable  working  values,  and  to  predetermine  breaking 
strengths. 

Ultimate-strength  Curves: 

40000 
(1)  R.  D.  Coombs  &  Co.:    - 


1  + 


1 


16,000  r 


(2)  Joint  Report  Crossing  Specifications :  54,000  —  180  - 


(3)  American  Bridge  Co. 


(4)  American  Railway 
Engineering  Association : 

Working-stress  Curve: 

(5)  R.  D.  Coombs  &  Co. 
(with  a  factor  of  safety  of  2.0) : 

or 
(with  a  factor  of  safety  of  2.5) : 


57,000  -  300^ 

39,000  -  150^ 
39,000  max. 

48,000  -  210^ 
42,000  max. 

20,000 


1  + 


1 


16,000  r2 
16,000 


1  + 


1 


16,000  r2 

Formulas  2,  3  and  4  shown  in  Fig.  31  were  not  issued  in  that 
form  by  their  authors,  but  in  terms  of  allowable  unit  stresses,  as 
follows : 

2.       18,000  -  60- 
19,000  -  100  - 
13,000  -  50  ^ 


4. 


13,000  max. 
16,000  -  70  - 
14,000  max. 


The  diagrams  shown  were,  therefore,  obtained  by  increasing 
these  allowable  units  to  their  apparent  ultimate  values.  The 
difficulty  in  comparing  the  usual  allowable-unit  formulas  arises 
from  their  variable  and  uncertain  factors  of  safety.  Such  formu- 
las were  rarely  intended  for  use  with  the  large  ratios  of  l/r  com- 


72 


POLE  AND  TOWER  LINES 


24.UUU 
23,000 
22,000 
21,000 
20,000 
19.000 
18,000 

Z4.UW 

23,000 
22,000 
21,000 
20,000 
19,000 
18,000 

17,000  -g 

a 
16,0002 
15,000  1 
14,000  S 
13.000  i 
12,000  1 

11,000-3 

10,000» 
9,000  | 
8,000  | 
7,000 
6,000 
5,000 
4,000 
3,000 
2,000 
1,000 

0 
ffl 

Graphical  Comparison  of  Various  Formulae 
for  Allowable  Working  Stresses  in  Steel  Columns 

"•--. 

x. 

X\ 

\ 

\ 

S 

—  -« 

•^: 

1  15,000 

cr 
00  14  nnn 

X 

~~'-s 

x>; 

\ 

\ 

*^« 

» 

^ 

^ 

<^- 

^ 

.000 

.pto 

oo> 

-V. 

fnit  Stresses  per 

!  I  !  1  1 

1W 

V 

Oapl 

^ 

5 

>^ 

$": 

v^ 

V 

s 

^ 

^5 

^ 

^ 

^ 

^txj 

^12 

boo-u 

ptoc 

£ 

<^ 

1 

^ 

£^\\ 

js 

^< 

^-, 

^x 

X, 

"^ 

~^- 

^ 

[^ 

^ 

X 

\: 

Xtl> 

x 

r^- 

-. 

U)1U,UW 

s 

•8  9.000 
|  8.000 
7,000 
6,000 
5,000 
4,000 
3,000 
2,000 
1,000 
0 

^~ 

^^ 

^ 

^ 

^v 

^ 

^ 

X 

^3,s 

^x 

^ 

^i 

^ 

^ 

<^ 

^^? 

^ 

X 

2 

V 

**•> 

-..^ 

\^ 

^~ 

^ 

^ 

^ 

^ 

X; 

*., 

~~ 

»-„ 

N 

SS 

§2 

1 

1 

\ 

•*•- 

\ 

\s 

\ 

""^ 

4? 

^ 

;<^ 

X; 

\ 

x 

\ 

\ 

^ 

ss 

s 

^ 

^ 

S 

\s 

\ 

X 

n 

^X 

^ 

\ 

S 

a 

10> 

K 

x 

f 

^ 

x 

^ 

!  23 

N 

s 

1 

\ 

^ 

B    10    20    30    40    50    60    70    80    90    100  110  120  130  140  150  160  170  180  190  200  210  2 
Ratio  -^r 

FlG.   34. 


-               17,LM>(                  OaVinrno  fTiv      ^i-ionifi  nnf  'na^ 

10.  15,200-  58  y 

11.15,000-57- 

r 

I    i          *_ 

2.  20,300  -  70  —  Chicago  Bridge  &  Iron  Wks. 
r      (Tank  Towers) 

L+36,OOOr2 

4.  17,100  —  57  —  Carnegie,  Jones  &Laughlin, 
r      Buffalo-Minneapolis 

5.  16,000-55  -  Bethlehem  Steel  Co. 
r 
•t  o  F^nn 

12.  12,500  —  42  — 

13.  11,300-35  — 
15,000 

11         Z2 

1  13,500r2 
17,000 

i   I          '2                Laughlin,  Phoenix  Bridge 

L  '  36,000r2          Co. 
12,000 

7.                .2            Boston 
1+36,000r2 

1  Il,000r2 
16,250 

1  18,000r2 

9.  16,000-60-  J.  A.L.  Waddell  ("De  Pon- 
r       tibus") 

16.               lz 
+  ll,000r2 

New  York  City,  Washing- 
ton Nat'l.  B'd.  Fire  Un- 
derwriters 

Passaic  Steel  Co. 


C.  E.  Fowler  ("Steel  Roofs 
&  B'ldgs.") 

H.  E.  Horton 


Chesapeake  &  Ohio  Ry., 
Norfolk  &  Western  Ry., 
Phila.  &  Reading  Ry., 
Penna.  R.  R.,  N.  Y.  C. 
(for  H'way  Bridges) 

Virginia  Br.  &  Iron  Co., 
Baltimore  &  Ohio  Ry., 
Chesapeake  &  Ohio  Ry., 
Long  Island  R.  R.,  Deep- 
water  Ry.,  Phila.  & 
Reading  Ry. 

Philadelphia 


DESIGN 


73 


mon  in  tower  work,  nor  were  the  conditions  of  loading  with  which 
they  were  used  as  unusual  as  those  encountered  in  transmission- 
line  construction.  In  other  words,  tower  members  are  less 
likely  to  approach  their  theoretically  perfect  condition  and, 
therefore,  should  have  a  high  factor  of  safety,  while  the  loading 
for  which  they  are  usually  figured  may  never  occur,  and  on  that 
account  they  should  have  a  low  factor. 

TABLE  14. — ULTIMATE  STRENGTHS  OF  TIMBER  IN  BENDING 


N.E.L.A.   Overhead 
Line  Construction 
Committee,  1911 

A.R.E.A.  Wooden 
Bridges  and  Trestles 
Committee,  1909 

. 

Lb.  per  square  inch 

Lb.  per  square  inch 

Port  Orford  cedar                   

6900 

Long-leaf  yellow  pine 

6600 

6500 

Douglas  fir  
Short-leaf  yellow  pine      

6000 
5700 

6100 
5600 

White  oak 

5700 

5700 

Chestnut  

5100 

Washington  cedar  
Idaho  cedar                                     .  . 

5100 
5100 

Redwood 

5100 

5000 

Bald  cypress  (heart  wood)  
Red  cedar 

4800 
4200 

4800 
4200 

Eastern  white  cedar 

3600 

Juniper 

3300 

Catalpa 

3000 

Spruce 

4800 

Western  hemlock 

5800 

It  should  be  noted  that  the  extension  of  the  ultimate-strength 
curve,  1,  to  values  of  l/r  of  300  is  not  intended  as  a  recommendation 
of  such  values,  but  rather  to  illustrate  the  decrease  of  strength. 
Further,  even  the  reduced  values  shown  will  not  always  be  ob- 
tained in  practice  owing  to  errors  of  design,  workmanship,  and 
injuries  from  handling. 


17.  19,000-100—  (—to  120")  Amer.  Bridge 

r  \r  Co. 

13,000-  50—  (—120^200) 
r  \r  ) 

13,000  Maximum 

18.  2°'°?P          H.B.  Seaman 


8,000r2 

19.  16,000-70  —Am.   Ry.,  Eng.  &  M.  W. 
r      Assn.,N.  Y.  C.     R.    R. 
Bos.  &  Me.   Ry.,  Cana- 
dian Pacific  Ry.,  Grand  Trunk  Ry.,  Maine 
Central  Ry.,  Mo.,  Kansas  &  Tex.  Ry.,  Nash., 


Chatt.  &  St.  L.  Ry.,  N.  Y.,  N.  H.  &  H.  Ry. 
St.  Louis  &  San  Fran.  Ry.,  St.  Louis 
&  Southwestern  Ry.,  Wabash  Ry.,  M.  S. 
Ketchum  ("Steel  Mill  Bldgs.");  C.  C.Sch- 
neider ("Structural  Design  of  Bldgs.") 

20.  18,000-  90  —  Southern  Ry. 

21.  15,000-  75  — L.  D.  Rights 


22.  12,000  \1-  (i200«  j.  R.  Worcester 

23.  Rule.    J.  R.  Worcester 


74  POLE  AND  TOWER  LINES 

The  logical  procedure  would  seem  to  be  to  assume  only  the 
loads  which  could  reasonably  be  expected,  i.e.,  the  ice  and  wind 
loads  with  very  moderate  broken-  wire  conditions,  to  prohibit 
excessive  values  of  l/r,  as  well  as  the  very  thin  sections,  and  to  use 
a  fairly  low  factor  of  safety. 

The  American  Railway  Engineering  Association  values  are 
for  squared  timbers  used  in  railroad  construction,  assumed  to  be 
of  good  commercial  quality,  and  neither  very  green  nor  seasoned. 
It  should  be  remembered  that  these  values  are  the  breaking 
strengths  of  fairly  good  lumber,  and  a  more  conservative  factor 
of  safety  must  be  assumed  than  would  be  necessary  in  more 
uniform  material.  Further,  pole  timber  is  not  squared  and  is 
more  likely  to  contain  crooked  grain,  knots  and  rot.  It  would 
be  more  consistent  to  use  lower  ultimate  strengths  for  poles 
than  the  N.E.L.A.  values,  and  a  smaller  factor  of  safety  than 
6.0,  which  is  that  used  therewith. 

Strength  of  Wooden  Poles. 

WEAKEST  POINT  IN  WOODEN  POLES 

(a)  Neglecting  Wind  on  Pole 

y  =  distance  of  weakest  section  below  load. 
d  =  diameter  of  pole  at  load. 
t  =  increase  in  diameter  per  inch  of  length. 
P  =  resultant  load. 

All  dimensions  in  inches. 


c  = 

I 

c 

ds 


_ 


dy  ~  0.0982(d  +  ty)* 

(d  +  ty)  -  3ty  =  0 

A 
y  ~  2t 

Let  di  =  diameter  of  pole  at  distance  y  below  the  load. 
Then 

t  ==dL--d 

y 

From  above  equation, 

_  d_  _          dy 

y      o*     a/ 


-  d) 


DESIGN  75 

,  -  d)y  =  dy 


That  is,  the  diameter  of  the  pole  at  the  weakest  section  is  one 
and  one-half  times  that  at  the  load.  When  the  diameter  at  the 
ground  line  is  less  than  one  and  one-half  times  the  diameter  at 
the  load,  the  weakest  section  is  at  the  ground  line. 

WEAKEST  POINT  IN  WOODEN  POLES 

(6)  Including  Wind  on  Pole 

y  =  distance  below  load  P.      « 
d  =  diameter  of  pole  at  load. 
t  =  increase  in  diameter  per  inch  of  length. 
a  =  distance  of  load  P  below  top  of  pole. 
w  =  wind  load  on  pole  per  inch  of  length. 

M  Py  w(y  +  a)2 

''   I    ~  0.098(d  +  tyY       2X0.098(d  +  ty)* 


2P       d  .  4P2       2Pd   ,   SPa   ,   d2      2da 


w  =  —  —  ~. — r  "&  —  *i       9 

w         t  \   w2         wt  w 

or 


"•S~T'          v\^ 


CHAPTER  V 
WOODEN  POLES 

The  total  number  of  wooden  poles  in  use  in  the  United  States  is 
probably  40,000,000,  while  the  yearly  additions  approximate 
4,000,000.  Of  these  the  majority  are  used  by  telephone,  tele- 
graph, and  railroad  companies.  The  greater  part  of  the 
4,000,000  new  poles  are  less  than  40  ft.  long,  and  such  poles 


FIG.  35. — Wooden  pole  lines,  60,000  volts. 

are  rarely  used  for  transmission  lines.  The  lengths  of  the 
poles  used  for  transmission  lines  are  increasing,  while  those  for 
other  purposes  are  decreasing.  In  the  East  the  timber  generally 
used  is  chestnut,  while  cedar  is  more  common  in  the  West.  For 
distribution  lines  not  only  the  length  of  the  poles  is  increasing, 

76 


WOODEN  POLES 


77 


but  also  the  number^  installed  per  year.     The  cost  of  poles  in- 
creases very  rapidly  with  increases  in  length. 

Approximately  90  per  cent,  of  the  timber  poles  used  are  either 
chestnut  or  cedar,  the  former  being  about  18  per  cent,  and  the 
latter  72  per  cent. 

Chestnut,  which  is  second  in  use  to  cedar,  is  durable  and  is 
stronger  than  cedar,  and  its  taper  is  not  excessive.  On  the  other 
hand,  it  is  heavier  and  harder,  it 
shrinks  and  checks  more  easily, 
and  is  not  so  straight  or  free  from 
knots. 

Cypress  is  durable,  but  its  size 
and  taper  often  make  it  unsuit- 
able for  pole  purposes.  Pine  is 
not  durable  and  is  heavy,  but  it 
grows  in  suitable  sizes. 

Douglas  fir,  spruce  and  red- 
wood are  durable  and  make  ex- 
cellent poles  when  of  suitable 
size.  The  consumption  of  the 


FIG.  36. — Ring  shakes  in  chestnut.          FIG.  37. — Cat-faces  in  chestnut. 

former  for  pole  purposes  is  increasing.     Redwood,  on  account  of 
its  size,  is  used  almost  exclusively  in  the  form  of  sawed  poles. 

The  useful  life  of  a  timber  pole,  in  contact  with  the  soil,  de- 
pends in  part  on  the  chemical  action  of  the  earth's  ingredients,  the 
attack  by  fungi,  and  on  the  ability  of  the  timber  to  resist  insects. 
Disintegration  will,  therefore,  advance  more  rapidly  in  some 
soils  than  in  others,  but  in  general  the  use  of  good  native  timber 


78  POLE  AND  TOWER  LINES 

for  local  use  will  be  found  advisable.  Decay  at  the  ground  line 
weakens  the  body  of  a  pole  until  this  critical  section  is  so  emaciated 
that  it  will  no  longer  sustain  its  load.  In  the  dry  season  this 
decayed  portion  is  much  in  the  nature  of  dry  tinder,  so  if  the 
pole  is  located  on  a  grassy  right-of-way,  grass  fires  may  char 
away  still  more  of  the  critical  section. 

Decay  and  Defects. — The  decay  of  wood  is  generally  due  to 
the  activities  of  certain  low  forms  of  plant  life  known  as  fungi, 
punk,  toadstools,  etc.  Bacteria  are  also  known  to  cause  decay, 
but  their  action  is  not  well  understood.  These  plants  have  their 
origin  in  minute  spores  borne  from  place  to  place  by  the  wind. 


FIG.  38. — Butt-rot  in  chestnut. 

Those  that  lodge  in  a  suitable  situation  for  growth,  which  may  be 
on  either  living  or  dead  timber,  germinate,  and  provided  the 
conditions  are  favorable,  at  once  attack  the  wood.  The  plants 
grow  with  great  rapidity,  sending  out  numerous  threads  which 
penetrate  the  wood  and  attack  the  contents  of  the  wood  cells  and 
finally  the  cell-walls. 

The  most  favorable  conditions  for  the  growth  of  fungi  and 
other  organisms  of  decay  are  an  abundant  food  supply,  heat, 
moisture,  and  air,  the  amount  of  each  depending  on  the  kind  of 
organism.  A  certain  amount  of  moisture  must  be  present  or  de- 
cay cannot  set  in.  Air  is  also  essential,  and  thus  may  be  explained 
the  lasting  qualities  of  wood  when  kept  perfectly  dry,  and  the 
perfect  state  of  preservation  of  wood  which  has  been  under  water 
for  long  periods,  moisture  being  lacking  in  the  first  case  and  air 
in  the  second.  Again,  if  the  wood  is  protected  by  a  germicide  or 


WOODEN  POLES  79 

antiseptic,  it  will  not  decay  At  the  butt  of  the  pole,  though 
moisture  is  present,  air  is  excluded,  while  above  the  ground  the 
pole  is  generally  dry.  Decay  begins  where  moisture  and  air  are 
both  present,  the  former  perhaps  being  drawn  by  capillary  attrac- 
tion from  the  ground. 

Since  the  decay  of  timber  is  due  to  the  attacks  of  wood-destroy- 
ing fungi,  and  since  the  most  important  condition  of  the  growth 
of  these  fungi  is  water,  anything  which  lessens  the  amount  of 
water  in  wood  aids  in  its  preservation. 

Cold,  or  extreme  heat,  will  prevent  the  growth  of  fungi, 
although  the  necessary  degree  of  the  latter  is  beyond  the  limits 


FIG.  39. — Ant-eaten  butt. 

of  natural  temperatures.  The  character  of  the  soil  may  have  a 
marked  effect  on  the  decay  of  timber,  owing  to  the  ability  of 
certain  soils,  like  heavy'  clay,  to  hold  water  or  to  discourage  in- 
sect life. 

The  decay  of  poles  before  their  installation  in  the  line  may  be 
of  several  kinds  and  together  with  the  several  kinds  of  cracks  or 
other  injuries  constitute  what  are  known  as  "defects."  The 
former  include  butt  rot,  heart  rot,  ring  rot,  and  rotten  knots; 
and  the  latter,  seasoning  checks,  wind  shakes  or  ring  shakes,  cat 
faces,  and  loose  knots. 

Since  in  designing  the  strength  of  a  pole  is  computed  as  that 
of  a  solid  homogeneous  cylindrical  section,  it  is  evident  that  pro- 
nounced defects  may  materially  affect  the  actual  strength. 
Therefore,  either  pole  specifications  and  inspection  must  be 


80  POLE  AND  TOWER  LINES 

strict  enough  to  eliminate  poles  whose  actual  strengths  are  not 
reasonably  close  to  their  assumed  values,  or  else  a  large  factor  of 
safety  must  be  employed  to  take  care  of  irregularities. 

Some  detailed  instructions  to  inspectors  would  seem  very 
desirable,  since  exactly  the  same  defect  may  have  an  entirely 
different  significance  in  two  locations  on  the  pole.  Thus  a 
rotten  heart,  very  common  in  cedar,  may  be  in  the  center  of  the 
cross-section  and,  therefore,  of  the  least  effect,  or  it  may  be  well 
toward  one  side.  A  wood  pole  is  not  greatly  affected  by  hollow- 
ing out  a  small  portion  at  the  center  of  the  cross-section,  but 
the  strength  is  decreased  by  any  loss  of  area  near  the  circum- 
ference or  any  reduction  of  diameter.  The  strength  is  pro- 
portional to  the  cube  of  the  diameter,  and  the  relative  strengths 
for  different  conditions  would  be: 

15-in.  sound  diameter  =  100% 

15-in.  pole,  5-in.  rotten  heart  at  center  =    99% 

15-in.  pole,  5-in.  rotten  heart  2^  in.  off  center  =    89% 
12-in.  sound  diameter  (13^  in.  decay)  =    51% 


Seasoning.  —  Under  present  methods  much  timber  is  rendered 
unfit  for  use  by  improper  seasoning.  When  exposed  to  the 
sun  and  wind  the  water  will  evaporate  more  rapidly  from  the 
outer  than  from  the  inner  parts  of  a  log  and  more  rapidly  from 
the  ends  than  from  the  sides.  The  evaporation  of  water  from 
timber  is  largely  through  the  ends.  The  evaporation  from  the 
other  surfaces  takes  place  very  slowly  out-of-doors,  with  greater 
rapidity  in  a  kiln.  The  rate  of  evaporation  differs  with  the 
kind  of  timber  and  its  shape.  Air-drying  out-of-doors  takes 
from  two  months  to  a  year,  the  time  depending  on  the  kind  of 
timber  and  the  climate.  As  the  water  evaporates  the  wood 
shrinks,  and  when  the  shrinkage  is  not  fairly  uniform  the  wood 
cracks.  When  wet  wood  is  piled  in  the  sun,  evaporation  may 
occur  with  such  unevenness  that  the  timber  splits  and  cracks 
so  badly  as  to  become  absolutely  useless.  Such  uneven  drying 
can  be  largely  prevented  by  careful  piling.  When  solid  piles 
are  placed  side  by  side  and  many  together,  the  air  cannot  cir- 
culate freely  between  the  timbers.  Open-crib  piles,  however, 
will  allow  free  air  circulation  even  when  closely  spaced.  For 
this  reason  green  timber  should  be  piled  in  as  open  piles  as 
possible,  as  soon  as  it  is  cut,  and  kept  so  until  it  is  air  dry.  No 
timber  should  be  treated  until  it  is  air  dry. 

Seasoning  is  ordinarily  understood  to  mean  drying,  but  it 


WOODEN  POLES  81 

really  involves  other  changes  besides  the  evaporation  of  water. 
It  is  very  probable  that  these  consist  in  changes  in  the  substances 
in  the  wood  fiber,  and  possibly  also  in  the  tannins,  resins  and 
other  incrusting  substances. 

One  of  the  first  steps  in  preparing  naturally  short-lived  timber 
for  preservative  treatment  is  to  season  it  properly.  More 
benefit  will  result  from  taking  care  of  the  short-lived  timbers  than 
from  treatment  of  those  with  longer  life. 

The  bark  should  be  peeled  from  poles  before  seasoning,  and 
particularly  from  those  that  are  to  be  treated,  as  the  inner 
bark  offers  considerable  resistance  to  impregnating  fluids  and 
if  not  removed  will  peel,  leaving  the  untreated  wood  exposed 
to  the  attack  of  fungi.  Bark  will  also  retard  and  almost  pre- 
vent seasoning.  Care  should  be  taken  in  handling  and  felling 
trees,  as  those  which  are  split  in  felling  or  are  otherwise  roughly 
handled  may  afterward  undergo  serious  checking.  Whether 
poles  are  to  receive  preservative  treatment  or  not,  there  can  be 
no  doubt  that  it  invariably  pays  to  season  them  properly  before 
putting  them  into  service.  Under  ordinary  conditions,  the 
life  of  a  well-seasoned  untreated  pole  should  be  at  least  30  per 
cent,  greater  than  that  of  an  untreated  green  pole. 

Poles  should  be  cut  from  sound  timber,  which  may  or  may  not 
be  live  timber.  Poles  cut  in  the  late  winter  or  spring  have 
immediately  before  them  the  best  period  for  seasoning,  but  late 
f alf  and  winter  offer  the  best  conditions  for  cutting,  and  facilitate 
the  cultivation  of  new  sprouts  from  the  winter-cut  stumps. 

Preservatives. — The  creosotes  are  employed  most  generally 
for  protecting  timber  against  decay,  and  they  are  apparently 
the  best  type  of  preservative.  The  terms  creosote  and  tar 
are  rather  general  expressions,  and  not  very  definitely  inter- 
preted by  the  ordinary  purchaser.  Briefly,  coal-tar  is  produced 
by  the  distillation  of  coal  and  is  obtained  from  two  distinct 
and  different  sources,  i.e.,  that  from  coke  ovens  and  that  from  gas 
works.  Crude  coal-tar  is  subjected  to  different  refining  proc- 
esses, and  yields  various  commercial  derivatives,  such  as  the 
different  grades  of  creosote,  carbolics,  naphthas,  etc.  The 
protective  effect  of  all  preservatives  is  due  to  their  exclusion  of 
water  and  to  their  antiseptic  or  poisonous  effect  on  the  fungi 
which  cause  decay.  In  order  to  protect  timber  permanently, 
it  is  necessary  that  the  preservative  be  maintained  either  as  an 
impervious  coating  on  the  surface  or  as  an  impregnation  through- 


82  POLE  AND  TOWER  LINES 

out  at  least  the  outer  layers  of  the  wood.  A  thin  permanent 
surface  coating  is  impracticable;  therefore,  to  obtain  successful 
results  the  preservative  should  be  injected  into  the  timber. 
The  deeper  the  penetration  and  the  more  insoluble  and  non- 
evaporative  the  injected  material,  the  more  successful  will  be 
the  treatment.  It  is  possible  to  inject  a  light  solution  to  greater 
depths  than  a  heavy  solution,  but  it  may  be  that  the  lighter 
solution,  which  may  contain  more  volatile  matter,  will  evaporate 
or  wash  out  more  readily  than  a  heavier  solution.  It  is  cus- 
tomary, however,  to  regard  the  greatest  penetration  as  the 
most  desirable,  and  to  grade  the  treatment  by  the  weight  of 
preservative  injected  into  a  given  volume  of  timber.  Such 
units  of  measurement,  however,  are  only  comparable  if  the 
qualities  of  the  injected  fluids  are  similar. 

In  addition  to  the  three  regular  grades  of  creosote  oil,  i.e.,  the 
A.R.E.A.  Nos.  1,  2  and  3,  the  specifications  of  which  are 
recognized  standards,  there  are  in  use  various  combinations  of 
creosote  and  coal-tar  as  well  as  a  number  of  proprietary  creosotes, 
or  creosote-tar  combinations.  Since  creosote  is  obtained  from 
tar,  and  is  a  part  of  crude  tar,  the  distinction  between  creosote,  as 
the  word  is  ordinarily  used,  and  crude  tar  is  very  indefinite. 
Undoubtedly  a  proper  impregnation  with  A.R.E.A.  No.  1 
creosote  is  the  most  economical  treatment  in  ultimate  result. 
Aside  from  this  grade  of  material,  however,  there  is  considerable 
difference  of  opinion  as  to  the  relative  merits  of  using  greater 
quantities  of  the  Nos.  2  or  3  grades  of  straight  creosote,  or  of 
using  creosote-coal-tar  combinations,  or  the  proprietary  solutions. 

Since  the  electric  companies  are  frequently  also  operators  of 
gas  plants,  it  is  probable  that  there  is  a  good  commercial  argu- 
ment in  favor  of  their  developing  the  combination  creosote-coal- 
tar  treatment,  even  though  theoretically  the  highest  grade 
treatment  is  ultimately  economical  for  absolutely  permanent 
construction.  In  other  words,  the  writer  is  of  the  opinion  that, 
since  such  companies  use  large  quantities  of  timber  for  in- 
stallations the  existence  of  which  is  not  permanent,  they  would 
be  justified  in  obtaining  a  less  effective  preservation  at  a  lower 
cost.  Further,  if  the  development  of  their  own  resources  would 
encourage  their  more  general  use  of  preservatives,  the  ultimate 
result  would  be  advantageous,  whether  or  not  the  kind  of  treat- 
ment for  any  particular  case  were  absolutely  the  best  possible. 

Gas-house  tar  and  coke-oven  tar  are  practically  alike  chem- 


WOODEN  POLES  83 

ically  but  differ  greatly  in  the  percentage  of  free  carbon,  the 
former  having  a  comparatively  high  percentage  and  the  latter 
usually  a  low  percentage.  High  free-carbon  tar,  whether  gas 
house  or  coke  oven,  should  not  be  used  as  an  addition  to  creosote 
oil.  If,  therefore,  the  combination  material  is  to  be  used,  only 
a  low-carbon  tar  should  be  allowed  in  the  creosote-coal-tar 
combination  preservative. 

Pressure  Treatment. — Turning  now  from  a  discussion  of  the 
materials  to  be  used  in  protecting  poles  from  decay  we  find  that 
several  processes  of  treatment  are  in  common  use;  the  high- 
pressure,  the  open-tank,  and  the  brush  treatment,  these  being 
stated  in  the  order  of  their  effectiveness. 

When  treated  by  the  high-pressure  method,  the  timber  is 
placed  in  metal  cylinders  and  subjected  to  a  steaming  or  heating 
process  followed  by  a  vacuum.  After  the  vacuum  has  been 
maintained  from  one  to  two  hours,  the  tank  is  completely  filled 
with  creosote  oil  and  pressure  is  applied  and  maintained  until 
the  specified  amount  of  creosote  has  been  forced  into  the  timber. 

Open-tank  Process. — The  open-tank  process  consists  in  treat- 
ing the  butts  of  the  poles  only.  Seasoned  timber  is  immersed 
in  a  tank  of  hot  preservative  and  kept  there  for  a  period  of  from 
one  to  three  hours.  The  timber  is  then  suddenly  transferred  to 
a  bath  at  atmospheric  temperature  and  kept  there  from  one  to 
three  hours  longer.  By  this  process  the  air  and  moisture  in  the 
cells  of  the  wood  are  first  expanded  and  some  driven  off,  while  in 
the  second  bath  the  air  and  moisture  contract  drawing  the  pre- 
servative liquid  into  the  timber. 

Brush  Treatment. — By  the  brush  method,  dry  seasoned  timber 
is  given  two  or  more  coats  of  hot  preservative  applied  with  three- 
or  four-knot  rubber-set  or  wire-bound  roofing  brushes.  The 
creosote  should  be  heated  to  about  200°F.  and  kept  at  that  tem- 
perature while  being  applied.  A  liberal  quantity  of  liquid  should 
be  used  and  it  should  be  well  brushed  into  all  crevices  in  the 
timber.  Before  applying  preservative,  the  poles  must  be  stripped 
of  bark,  inner  skin,  or  dirt,  and  in  fact  should  be  scrubbed  clean. 
Sufficient  time  should  elapse  between  the  application  of  the 
different  coats  for  the  preceding  one  to  be  absorbed ;  not  less  than 
one-day  intervals  are  generally  satisfactory.  Poles  should  not  be 
used  for  two  or  three  days  after  treatment.  In  general  it  is  more 
economical,  both  in  labor  and  in  the  efficiency  of  the  operation,  to 
treat  poles  while  they  are  in  temporary  storage,  although  some 


84  POLE  AND  TOWER  LINES 

benefit  is  unquestionably  derived  from  even  a  cold  application 
at  the  site. 

Whenever  brush  treatment  is  employed  the  entire  butt  should 
be  coated  up  to  about  2  ft.  above  the  ground  line.  In  addition, 
all  crossarm  gains,  roofs  and  bolt  holes  should  be  painted  with 
preservative. 

SPECIFICATIONS  FOR  WOOD  POLES 

The  purchaser  shall  have  the  right  to  make  such  inspection  of  the 
poles  as  may  be  desired.  The  inspector  representing  the  purchaser 
shall  have  the  power  to  reject  any  pole  which  is  defective  in  any  respect. 
Inspection,  however,  shall  not  relieve  the  contractor  from  the  re- 
sponsibility of  furnishing  proper  poles. 

Any  imperfect  poles  which  may  be  discovered  before  their  final  accept- 
ance shall  be  replaced  immediately  upon  the  order  of  the  purchaser, 
even  though  the  defects  may  have  been  overlooked  by  the  inspector. 

Poles  shall  be  subject  to  inspection  by  the  purchaser,  either  in  the 
woods  where  the  trees  are  felled  or  at  any  point  of  shipment  or  delivery. 
Any  poles  failing  to  meet  the  requirements  of  these  specifications  may  be 
rejected. 

Seasoned  poles  shall  have  preference  over  green  poles,  provided  they 
have  not  been  held  for  seasoning  long  enough  to  have  developed  any  of 
the  timber  defects  hereinafter  referred  to.  All  poles  shall  be  reasonably 
straight,  well  proportioned  from  butt  to  top,  shall  have  both  ends 
squared,  the  bark  peeled  and  all  knots  and  limbs  closely  trimmed. 

DEAD  POLES. — No  dead  poles  and  no  poles  having  dead  streaks 
covering  more  than  one-quarter  of  their  surface  shall  be  accepted  under 
these  specifications.  Poles  having  dead  streaks  covering  less  than  one- 
quarter  of  their  surface  shall  have  a  circumference  greater  than  otherwise 
required.  The  increase  in  the  circumference  shall  be  sufficient  to  afford 
a  cross-sectional  area  of  sound  wood  equivalent  to  that  of  sound  poles  of 
the  same  class. 

TWISTED,  CHECKED  OR  CRACKED  POLES. — No  cracked  poles,  no  poles 
containing  large  seasoning  checks,  and  no  poles  having  more  than  one 
complete  twist  for  twenty  (20)  ft.  in  length  shall  be  accepted  under  these 
specifications. 

CROOKED  POLES. — No  poles  having  a  short  crook  or  bend,  a  crook  or 
bend  in  two  planes,  or  a  reverse  crook  or  bend,  shall  be  accepted  under 
these  specifications.  The  amount  of  sweep  measured  between  the  six 
(6)  ft.  mark  and  the  top  of  the  pole  shall  not  exceed  one  (1)  in.  for 
every  six  (6)  ft.  of  length. 

MISCELLANEOUS  DEFECTS. — No  poles  containing  sap  rot,  evidence  of 
internal  rot  as  disclosed  by  careful  examination  of  black  knots,  hollow 
knots,  woodpecker's  holes,  or  plugged  holes,  and  no  poles  showing 


WOODEN  POLES 


85 


evidences  of  having  been  eaten  by  ants,  worms  or  grubs  shall  be  accepted 
except  that  poles  containing  worm  or  grub  marks  below  the  six  (6)  ft. 
mark  may  be  accepted. 

CAT  FACES. — No  poles  having  "cat  faces,"  unless  the  latter  are  small 
and  perfectly  sound  and  the  poles  have  an  increased  diameter  at  the 
"cat  face,"  and  no  poles  having  "cat  faces"  near  the  six  (6)  ft.  mark 
or  within  ten  (10)  ft.  of  their  tops  shah1  be  accepted. 

WIND  SHAKES. — No  poles  shall  have  cup  shakes  (checks  in  the  form 
of  rings)  containing  heart  or  star  shakes  which  enclose  more  than  ten 
(10)  per  cent,  of  the  area  of  the  butt. 

BUTT  ROT. — No  poles  shall  have  butt  rot  covering  more  than  ten  (10) 
per  cent,  pf  the  total  area  of  the  butt.  If  butt  rot  is  present  it  must  be 
located  close  to  the  center  in  order  that  the  pole  may  be  accepted. 

KNOTS. — Large  knots,  if  sound  and  trimmed  close,  shall  not  be  con- 
sidered a  defect.  No  poles  shall  contain  loose,  hollow  or  rotten  knots. 

DEFECTIVE  TOPS. — Poles  having  tops  of  the  required  dimensions  shall 
not  be  accepted  unless  the  tops  are  sound.  Poles  having  tops  one  (1) 
in.  or  more  in  excess  of  the  required  circumference  may  contain  one 
(1)  pipe  rot  not  more  than  one-half  (0.5)  in.  in  diameter.  Poles  with 
double  tops  or  double  hearts  shall  be  free  from  rot  where  the  two  parts  or 
hearts  join. 

DEFECTIVE  BUTTS. — No  poles  containing  ring  rot  (rot  in  the  form  of 
a  complete  or  partial  ring)  shall  be  accepted  under  'these  specifications. 

Poles  having  hollow  hearts  may  be  accepted  under  the  conditions 
shown  in  the  following  table : 

TABLE  15 


Average  diameter 


Add  to  butt  requirement  (circumference) 


of  rot 

25-  and  30-ft.  poles 

35-,  40-  and  45-ft.  poles 

50-,  55-,  60-  and  65-ft. 
poles 

2  in. 

Nothing 

Nothing 

Nothing 

3  in. 

lin. 

Nothing 

Nothing 

4  in. 

2  in. 

Nothing 

Nothing 

5  in. 

3  in. 

lin. 

Nothing 

Gin. 

4  in. 

2  in. 

1  in. 

7  in. 

Reject 

4  in. 

2  in. 

8  in. 

Reject 

6  in. 

3  in. 

9  in. 

Reject 

Reject 

4  in. 

10  in. 

Reject 

Reject 

5  in. 

11  in. 

Reject 

Reject 

7  in. 

12  in. 

Reject 

Reject 

9  in. 

13  in. 

Reject 

Reject 

Reject 

Scattered  rot,  unless  it  is  near  the  outside  of  the  pole,  will  be  considered 
being  the  same  as  heart  rot  of  equal  area. 


86 


POLE  AND  TOWER  LINES 


DIMENSIONS. — The  dimensions  of  the  poles  shall  be  not  less  than  the 
values  given  in  the  following  tables,  the  "top"  measurement  being  the 
circumference  at  the  top  of  the  pole  and  the  "butt"  measurement  the 
circumference  six  (6)  ft.  from  the  butt. 

The  dimensions  specified  for  the  six  (6)  ft.  mark  shall  be  required  in 
all  cases,  but  the  top  circumferences  may  differ  from  those  shown  in  the 
following  tables  by  not  more  than  one-half  (0.5)  in.  No  pole  shall  be 
more  than  six  (6)  in.  longer  or  three  (3)  in.  shorter  than  the  length  for 
which  it  is  accepted.  If  any  pole  is  more  than  six  (6)  in.  longer  than  is 
required  it  shall  be  cut  back. 


TABLE  16. — CHESTNUT 


Circumferences  of  poles  in  inches 


Cl 

asses 

Length  of 
poles  (ft.) 

A 

B 

C 

Top 

(in.) 

6  ft.  from 
butt  (in.) 

Top 
(in.) 

6  ft.  from 
butt  (in.) 

Top 

(in.) 

6  ft.  from 
butt  (in.) 

25 

20 

30 

30 

24 

40 

22 

36 

20 

33 

35 

24     . 

43 

22 

40 

20 

36 

40 

24 

45 

22 

43 

20 

40 

45 

24 

48 

22 

47 

20 

43 

50 

24 

51 

22 

50 

20 

46 

55 
60 

22 
22 

54 
57 

22 
22 

53 
56 

20 

49 

65 

22 

60 

22 

59 

70 

22 

63 

22 

62 

75 

22 

66 

22 

65 

80 

85 

22 
22 

70 
73 

22 

22 

69 

72 



90 

22 

76 

22 

75 

WOODEN  POLES 


87 


TABLE  17. — EASTERN  WHITE  CEDAR 


Circumferences  of  poles  in  inches 


Cl 

asses 

Length  of 
poles  (ft.) 

A 

B 

C 

Top 
(in.) 

6  ft.  from 
butt  (in.) 

Top 

(in.) 

6  ft.  from 
butt  (in.) 

Top 
(in.) 

6  ft.  from 
butt  (in.) 

25 

22 

32 

18% 

30 

30 

24 

40 

22 

36 

18% 

23 

35 

24 

43 

22 

38 

18% 

36 

40 

24 

47 

22 

43 

18% 

40 

45 

24 

50 

22 

47 

18% 

43 

50 

24 

53 

22 

50 

18% 

46 

55 

24 

56 

22 

53 

18% 

49 

60 

24 

59 

22 

56 

TABLE  18. — WESTERN  WHITE  CEDAR,  RED  CEDAR,  WESTERN 
CEDAR,  IDAHO  CEDAR 

Circumferences  of  poles  in  inches 


Length  of 
poles  (ft.) 

Classes 

A 

B 

C 

Top 

(in.) 

6  ft.  from 
butt  (in.) 

Top 
(in.) 

6  ft.  from 
butt  (in.) 

Top 

(in.) 

6  ft.  from 
butt  (in.) 

20 

28 

30 

25 

28 

22 

26 

22 

28 

32 

25 

30 

22 

27 

25 

28 

34 

25 

31 

22 

28 

30 

28 

37 

25 

34 

22 

30 

35 

28 

40 

25 

36 

22 

32 

40 
45 

28 
28 

43 
45 

25 
25 

38 
40 

22 
22 

34 
36 

50 

28 

47 

25 

42 

22 

38 

55 

28 

49 

25 

44 

22 

40 

1  60 

28 

52 

25 

46 

22 

41 

65 

28 

54 

25 

48 

22 

43 

SAWED  REDWOOD  POLES 

The  material  desired  under  these  specifications  consists  of  poles  of 
redwood  (Sequois  Sempervirens)  sawed  to  shape  as  hereinafter  set 
forth. 

QUALITY  OF  TIMBER  AND  WORKMANSHIP. — All  poles  shall  be  of  sound 
No.  1  Common  Redwood;  and  shall  be  reasonably  straight  and  well 
sawn. 


88 


POLE  AND  TOWER  LINES 

TABLE  19. — SAWED  REDWOOD 


Dimensions  in  inches 

Classes 

(ft.) 

A 

B 

Top  (in.) 

Butt  (in.) 

Top  (in.) 

Butt  (in.) 

24 

6X6 

6X6 

4X6 

4X6 

25 

7  X7 

10  X  10 

6X6 

9X9 

30 

7X7 

11  X  11 

6X6 

10  X  10 

35 

7X7 

12  X  12 

6X6 

11  X  11 

40 

7X7 

13  X  13 

6X6 

12  X  12 

45 

7X7 

14  X  14 

6X6 

13  X  13 

50 

7X7 

15^  X  15>£ 

6X6 

14  X  14 

The  sectional  dimensions  of  the  sawn  poles  shall  not  be  more  than 
one-quarter  (>£)  in.  under,  or  three-quarters  (%}  in.  over,  the  dimen- 
sions specified  in  the  above  table.  No  poles  shall  be  more  than  three 
(3)  in.  longer  or  shorter  than  the  lengths  required  in  the  above  table. 

SAP  WOOD. — No  poles  shall  have  sap  wood  covering  more  than  four  (4) 
per  cent,  of  the  area  of  all  the  surfaces.  No  pole  shall  have  sapwood 
for  a  distance  of  more  than  eight  (8)  ft.  from  the  top.  No  sapwood  shall 
be  deeper  than  one  (1)  in.  at  any  point. 

KNOTS. — In  4"  X  6"  poles  sound  knots  with  a  diameter  smaller  than 
one  (1)  in.  may  be  present  in  any  number.  No  4"X  6"  pole  shall  be 
accepted  which  contains  in  each  five  (5)  superficial  ft.  more  than  one 
sound  knot  having  a  diameter  of  one  (1)  in.  or  more,  or  which  con- 
tains any  knots  with  a  diameter  greater  than  one  and  one-half  (1>^)  in. 

All  other  sizes  of  poles  covered  by  these  specifications  may  contain 
any  number  of  sound  knots  with  a  diameter  smaller  than  one  and  one- 
half  (13^)  in.  No  pole  shall  be  accepted  which  contains  in  each  five  (5) 
superficial  ft.  more  than  one  sound  knot  having  a  diameter  of  one  and 
one-half  (1>^)  in.  or  more,  or  which  contains  any  knots  of  a  diameter 
greater  than  two  and  one-half  (2^)  in. 

NOTE. — Where  diameters  are  specified  in  connection  with  knots  a 
knot  shall  be  rated  on  the  basis  of  its  average  diameter. 


SPECIFICATIONS  FOR  CREOSOTED  YELLOW-PINE  POLES 

These  specifications  shall  apply  to  Classes  A,  B  and  C  poles  of  sputhern 
yellow  pine  treated  with  dead  oil  of  coal  tar. 

QUALITY  OF  POLES. — All  poles  shall  be  sound  southern  yellow  pine 
(longleaf ,  shortleaf,  or  loblolly  yellow  pine)  squared  at  the  buft,  reason- 
ably straight,  well  proportioned  from  butt  to  top,  peeled  and  'with 


WOODEN  POLES 


89 


knots  trimmed  close.  All  poles  shall  be  free  from  large  or  decayed  knots. 
All  poles  shall  be  cut  from  live  timber. 

It  is  desired  that  all  poles  be  well  air-seasoned  before  treatment  and 
such  poles  shall  be  treated  in  accordance  with  the  requirements  for 
treating  seasoned  timber  contained  in  the  "  Specifications  for  Creosoting 
Timber"  hereinafter  referred  to.  The  poles  shall  not  be  held  for  season- 
ing, however,  up  to  the  point  where  local  experience  shows  that  sapwood 
decay  would  begin.  Unseasoned  poles  shall  be  treated  in  accordance 
with  the  requirements  for  treating  unseasoned  timber  contained  in  the 
above-mentioned  specifications. 

All  poles  shall  be  sufficiently  free  from  adhering  inner  bark  before 
treating  to  permit  the  penetration  of  the  oil.  If  the  inner  bark  is  not 
satisfactorily  removed  when  the  pole  is  peeled,  the  pole  shall  be  either 
shaved  or  allowed  to  season  until  the  inner  bark  cracks  and  tends  to 
peel  from  the  pole. 

DIMENSIONS. — The  dimensions  of  the  poles  shall  be  not  less  than  those 
given  in  the  following  table. 

TABLE  20. — CREOSOTED  YELLOW  PINE 


Circumference  of  pole  in  inches 

Classes 

Length  of 
pole  (ft.) 

A 

B 

C 

Top 

(in.) 

6  ft.  from 
butt  (in.) 

Top 
(in.) 

6  ft.  from 
butt  (in.) 

Top 

(in.) 

6  ft.  from 
butt  (in.) 

25 

22 

33 

20 

30 

18 

28>i 

30 

22 

35 

20 

32 

18 

30H 

35 

22 

38 

20 

34 

18 

32 

40 

22 

40 

20 

36 

18 

34 

45 

22 

42^ 

20 

38 

18 

36 

50 

22 

44^ 

20 

40 

18 

38 

55 

22 

47 

20 

42^ 

18 

40 

60 

22 

49 

20 

44^ 

18 

42 

65 

22 

51 

20 

47 

18 

44 

70 

22 

53 

20 

49 

18 

46 

75 

22 

55 

20 

51 

80 

22 

57 

Framing  of  Poles. — Before  the  poles  are  treated  with  creosote  they 
shall  be  framed,  unless  otherwise  ordered,  in  the  following  manner  and  as 
shown  in  drawing  No.  (  ). 

The  top  of  each  pole  shall  be  roofed  at  an  angle  of  ninety  (90)  degrees. 

All  Class  A  poles  shall  have  eight  (8)  gains,  all  Class  B  poles  shall  have 
four  (4)  gains  and  all  Class  C  poles  shall  have  two  (2)  gains. 

The  gains  shall  be  located  on  the  side  of  the  pole  with  the  greatest 


90  POLE  AND  TOWER  LINES 

curvature,  and  on  the  convex  side  of  the  curve.  The  faces  of  all  gains 
shall  be  parallel. 

Each  gain  shall  be  four  and  one-quarter  (434)  in.  wide  and  one-half 
(Y^)  in.  deep  and  twenty-four  (24)  in.  center  to  center.  The  center  of 
the  top  gain  shall  be  ten  (10)  in.  from  the  apex  of  the  gaWe.  A  twenty- 
one  thirty-second  (2>£2)  in.  hole  shall  be  bored  through  the  pole  at  the 
center  of  each  gain  perpendicular  to  the  plane  of  the  gain. 

INSPECTION. — The  quantity  of  dead  oil  of  coal  tar  forced  into  the 
poles  shall  be  determined  by  tank  measurements  and  by  observing  the 
depth  of  penetration  of  the  oil  into  the  pole.  If  the  poles  have  more 
than  one  and  one-half  (1^)  in.  of  sap  wood,  the  depth  of  penetration 
shall  be  not  less  than  one  and  one-half  (1^)  in.  If  the  sap  wood  is  less 
than  one  and  one-half  (1^)  in.  thick,  the  dead  oil  of  coal  tar  shall  pene- 
trate through  the  sap  wood  into  the  heartwood. 

The  depth  of  penetration  shall  be  determined  by  boring  the  pole  with  a 
one  (1)  in.  auger.  The  right  is  reserved  to  bore  two  holes  at  random 
about  the  circumference  for  this  purpose,  one  hole  to  be  five  (5)  ft. 
from  the  butt  and  one  hole  ten  (10)  ft.  from  the  top.  After  inspection 
each  test  hole  shall  be  filled  first  with  hot  dead  oil  of  coal  tar  and  then 
with  a  close-fitting  creosoted  wooden  plug. 

The  rejection  of  any  pole  because  of  insufficient  penetration  shall  not 
preclude  its  being  retreated  and  again  offered  for  inspection. 

Design  of  Wood  Poles. — If  in  Fig.  40,  which  represents  a 
standard  Class  A  chestnut  pole,  we  assume  for  the  present  that 
all  the  bending  loads  are  represented  by  one  load,  P  of  1200  lb., 
35  ft.  from  the  ground,  the  bending  moments  and  unit  stresses 
at  sections  X2,  XI,  and  at  the  ground  line,  will  be  as  follows: 

At  X2, 

M  =  18.25  ft.  X  12  in.  X  1200  lb.  =  262,800  in.-lb. 
S  =  0.0982  X  123  =  169.69 

F  =  M/S  =  1550  lb.  per  square  inch 

At  XI, 

M  =  24.5  X  12  X  1200  =  352,800  in.-lb. 

S  =  0.0982  X  13.123  =  220.75 

F  =  M/S  =  1600  lb.  per  square  inch 

At  ground, 

M  =  35  X  12  X  1200  =  504,000  in.-lb. 

S  =  0.0982  X  153  =  331.42 

F  =  M/S  =  1530  lb.  per  square  inch 

It  should  be  noted  therefore,  that  the  point  of  greatest  stress  is 
not  necessarily  at  the  ground  line  but  may  be  at  some  section  above 


WOODEN  POLES 


91 


the  ground.  If  the  pole  under  consideration  were  disproportion- 
ally  heavy  at  the  butt,  any  computations  made  at  the  ground  line 
might  be  quite  erroneous,  although  the  difference  in  the  example 
given  is  negligible.  This  condition  results  from  the  fact  that  the 
unit  stress  "at  any  point  depends  on  the  distance  from  the  load 
and  on  the  diameter  of  the  pole  at  that  point.  Provided  there 
are  no  serious  defects  in  a  pole  which  may  make  some  particular 
point  unusually  weak  it  will,  in  theory,  break  at  the  point  where 
the  diameter  is  1.5  times  the  diameter  at  the  point  where  the 
load  is  applied.  Therefore,  poles  may  or  may  not  fail  at  the 


0 


A           > 
J       "o 

1  ^a 

; 

~  -  3  1 

x. 

3.1 

r 

„ 

• 

• 

1200 


FIG.  40. 


ground  line  depending  on  the  taper.  Further,  if  the  butt 
diameter  exceeds  the  above  critical  diameter  the  pole  may 
experience  some  decay  at  the  butt  without  becoming  any  weaker. 

Referring  to  Fig.  41,  if  the  following  wires  are  to  be  carried, 
with  a  minimum  clearance  of  30  ft.,  and  a  maximum  stress  in  the 
wires  of  0.9  of  the  elastic  limit,  we  have,  if  a  200-ft.  span  is 
assumed, 

One  ^g-m-  Siemens-Martin  galvanized  stranded  steel. 

Three  No.  1  hard-drawn  stranded  copper,  33,000  volts. 

Span  200  ft. 

Normal     sag  =  1  ft.  3  in.     Normal  tension       =  1020  Ib. 

Maximum  sag  =  2ft.  0  in.     Maximum  tension  =  1960  Ib. 

Elastic  limit,  No.  1  cable  =  2180  Ib. 

Wind  pressure  on  wires: 

%-in.  ground  wire  =  0.917  Ib.  X  200  ft.  =  183  Ib. 

No.  1  power  wire  =  0.885  Ib.  X  200  ft.  =  177  Ib. 


92 


POLE'  AND  TOWER  LINES 


The  bending  moment  at  the  ground,  for  transverse  loading, 
straight-line  poles  and  no  broken  wires,  is, 

Ground  wire,  183  Ib.  X  37  ft.  =    6,770  ft.-lb. 

Power  wires,  177  Ib.  X  34.5  ft.  =    6,100  ft.-lb. 

Power  wires,  177  Ib.  X  2  X  32  ft.  =  11,330  ft.-lb. 

07    C2 

Wind  on  pole,  0.9  sq.  ft.  X  13  Ib.  X  -g-  ft.  =    8,225  ft.-lb. 
Bending  moment  =  32,425  ft.-lb. 

f^__l 

-T— - 


1 


r~t. 


-i- 


••-  so 


^— f- 


.. 
Cleara 


:-15 


L 


FIG.  41. — 44-ft.  pole. 


The  shear  on  the  pole  is, 
Ground  wire,  183  Ib. 

Three  power  wires,    531  Ib. 
Wind  on  pole,  438  Ib. 


and 


Total  shear,  1152  Ib. 

32,425 
1152 


28.1 


WOODEN  POLES  93 

Or  the  load  is  equivalent  to  a  single  load,  P  =  11521b.,  28.1  ft. 
above  the  ground. 

If  the  weakest  section  of  the  pole  is  assumed  as  being  at  the 
ground  line,  which  is  usually  not  correct,  the  unit  stress  under  the 
first  condition  of  loading  is, 

M       SI 
M  —  - 

c 

32,425  ft.-lb.  X  12  in. 
~T~ 

c 

I        1  153 

-  =  —  X  diam.3  (approx.)  =  —  =  337.5 

C          -LU  J.U 

S  =  1150  Ib.  per  square  inch,  bending  stress. 

To  obtain  the  maximum  unit  bending  stress  in  the  pole: 
Center  of  gravity  of  wire  loads  below  top  of  pole: 

183  Ib.  X  6  in.  =  1,098  in.-lb. 
177  Ib.  X  36  in.  =  6,372  in.-lb. 
354  Ib.  X  66  in.  =  23,364  in.-lb. 

714  Ib.  30,834  in.-lb. 

30,834  in.-lb.  -f-  714  Ib.  =  43  in.  below  top  of  pole. 

The  location  of  the  point  of  maximum  stress  below  the  center 
of  gravity  of  wire  loads  can  be  found  from  the  formula,  page  75 : 


2P       d 


In  this  case 

P  =  wind  on  wires  =  714  Ib. 
w  =  wind  per  inch  of  pole  =  1  Ib. 
di  =  diam.  of  pole  at  top  =  7.4  in. 
dz  =  diam.  of  pole  6  ft.  0  in.  above  butt  =  15  in. 
dz  —  diam.  of  pole  at  point  of  maximum  stress. 
t  =  increase     in     diameter     per     inch    of     length 

15  -  7.4         _!_ 
~  38ft.  X  12  ~  60 
d  =  diam.  of  pole  at  load  P  =  8  in. 
a  =  dist.  of  load  P  below  top  of  pole  =  43  in. 


94  POLE  AND  TOWER  LINES 

Substituting  these  in  the  above  formula: 

y  =  304  in.  =  25  ft.  4  in. 

The  maximum  stress  occurs,  therefore,  25  ft.  4  in.  below  the 
center  of  gravity  of  the  wire  loads,  or  28  ft.  11  in.  below  the 
top  of  the  pole. 

d3  =  d  +  ty  =  8  in.  +  ^  X  304  =  13  in. 
Maximum  stress  in  pole 

M     P  a2 

'' 


_  0.098d33 

c 

714  X  304  -f  *  (304  +  43)2 

Zi 

~  0.098  X  133 

277  260 
=    01  g  o     =  1290  Ib.  per  square  inch. 

'  ZlO.O 

As  the  breaking  strength  for  a  chestnut  pole  is  5100  Ib.  per 
square  inch,  the  factor  of  safety  is  T^T:  =  4. 


The  stress  at  the  ground  line  equals 

714  X  (450  -  43)  +  — 


0.098  X  (14.9)3 

391  848 

-  =  1200  Ib.  per  square  inch. 


The  bending  moment  at  the  ground  for  a  pole  at  a  5°  corner, 
is  found  as  follows: 

Maximum  wire  tension  =  1960  Ib. 

Component  due  to  corner  (Fig.  32)  =  0.10  tension 
or 

1960  Ib.  X  0.10  =  195  Ib.  per  wire. 

Wind  on  wires  and  pole 

(same  as  before)  =  32,425  ft.-lb. 

195  Ib.  X  37  ft.  =  7,215  ft.-lb. 

195  Ib.  X  34.5  ft.  =  6,725  ft.-lb. 

195  Ib.  X  2  X  32  ft.  =  12,480  ft.-lb. 

Total  bending  moment  =  58,845  ft.-lb. 


WOODEN  POLES 

1152  Ib.  wind  oh  wires  and  pole 
780  =  195  X  4  wires,  corner  loading 


95 


1932  Shear 


and 


58,845 
1932 


=  30.4  ft. 


FIG.  42.  —  Corners  on  single  pin  insulators. 

Therefore 
P  =  1932  Ib.,  equiv.  load  30.4  ft.  above  ground 


8  =       > 


sq' 


In  Fig.  42  is  shown  a  one-circuit  wood-pole  line,  in  which  the 
wires  turn  rather  sharp  corners  on  single-pin  insulators,  a  prac- 
tice which  is  objectionable.  The  illustration  shows  considerable 
right-of-way  clearing,  but  it  appears  that  the  poles  will  pre- 
sumably not  be  subjected  to  very  severe  wind  loads  on  account  of 


96 


POLE  AND  TOWER  LINES 


the  shelter  afforded  by  adjoining  timber.  It  is  also  evident 
that  a  very  wide  clearing  would  be  ne'cessary  to  entirely  protect 
the  line  from  adjoining  trees. 


FIG.  43. — Design  for  wooden  A-frame. 

A-frames  and  H-frames. — Timber  A-frames  composed  of  two 
poles  spliced  together  at  the  top  and  with  their  butts  separated 
transverse  to  the  line  are  useful  chiefly  where  large  timber  is  ex- 


WOODEN  POLES 


97 


pensive,  as  such  construction  permits  the  use  of  slender  poles, 
one  of  which  would  not  have  sufficient  strength.  These  frames 
have  not  been  used  to  any  considerable  extent,  however,  in  this 
country.  In  the  direction  of  the  line,  the  strength  is  twice  that 
of  the  single  poles,  while  it  is  considerably  greater  in  a  transverse 
direction,  the  amount  depending  largely  on  the  bracing  provided 


FIG.  44. — One-circuit  H-frame. 


to  prevent  buckling  of  each  pole.  Except  at  corners,  these  frames 
are  relatively  too  strong  across  the  line  as  compared  with  their 
strength  in  the  direction  of  the  line. 

The  H-frame,  on  the  other  hand,  while  having  less  theoretical 
strength  across  the  line,  is  a  useful  type  of  construction,  par- 
ticularly for  heavy  lines  in  bad  ground.  Its  width  at  the  top 
permits  a  larger  number  of  wires  per  crossarm,  while  utilizing 
the  strength  of  the  arms  as  simple  beams  instead  of  cantilevers. 

In  Fig.  44  is  shown  a  one-circuit  H-frame,  consisting  of  two 
light  timber  poles.  This  is  typical  of  the  characteristic  usefulness 

7 


98  POLE  AND.  TOWER  LINES 

of  an  H-frame  in  that  two  slender  poles  can  be  used  to  provide 
adequate   strength.     On   account    of    their   strength   H-frames 


FIG.  45. — H-frame  crossing,  metal  grounding  arms. 

may  also  be  employed  to  support  heavy  lines,  although  they 
are  more  frequently  used  for  heavy  telephone  and  telegraph 
trunk  lines  than  for  transmission  lines  (Fig.  86). 


WOODEN  POLES 


99 


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POLE  AND  TOWER  LINES 


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M 

CHAPTER  VI 
STEEL  POLES  AND  TOWERS 

OPEN-HEARTH  STEEL 

Manufacturers'  Standard  Specifications.    Issue  of  Feb.  6,  1914. 

(Abstracts') 

All  tests  and  inspections  shall  be  made  at  the  place  of  manufacture 
prior  to  shipment. 

The  tensile  strength,  limit  of  elasticity  and  ductility  shall  be  deter- 
mined from  a  standard  test  piece  cut  from  the  finished  material. 

The  elongation  shall  be  measured  on  an  original  length  of  8  in. 
Rivet  rounds  and  small  bars  shall  be  tested  full  size  as  rolled. 

Two  tests  pieces  shall  be  taken  from  each  melt  or  blow  of  finished 
'material — one  for  tension  and  one  for  bending — but  in  case  either  test 
develops  flaws,  or  the  tensile  test  piece  breaks  outside  of  the  middle 
third  of  its  gaged  length,  it  may  be  discarded  and  another  test  piece 
substituted  therefor. 

Material  which  is  to  be  used  without  annealing  or  further  treatment 
shall  be  tested  in  the  condition  in  which  it  comes  from  the  rolls.  When 
material  is  to  be  annealed  or  otherwise  treated  before  use,  the  specimen 
representing  such  material  shall  be  similarly  treated  before  testing. 

Finished  bars  shall  be  free  from  injurious  seams,  flaws  or  cracks,  and 
have  a  workmanlike  finish. 

MAXIMUM  PHOSPHORUS. — 0.10  per  cent. 

RIVET  STEEL.— Ultimate  strength,  48,000  to  58,000  Ib.  per  square 
inch.  Elastic  limit,  not  less  than  one-half  the  ultimate  strength. 

1  400  000 

Percentage  of   elongation,  -  — •     Bending  test,   180°  flat 

ultimate  strength 

on  itself,  without  fracture  on  outside  of  bent  portion. 

RAILWAY  BRIDGE  GRADE.— Ultimate  strength,  55,000  to  65,000  Ib. 
per  square  inch.  Elastic  limit,  not  less  than  one-half  the  ultimate 

strength.     Percentage  of  elongation,  —  — •     Bending   test, 

ultimate  strength 

180°  to  a  diameter  equal  to  thickness  of  piece  tested,  without  fracture 
on  outside  of  bent  portion. 

MEDIUM  STEEL. — Ultimate  strength,  60,000  to  70,000  Ib.  per  square 
inch.  Elastic  limit,  not  less  than  one-half  the  ultimate  strength. 

103 


104 


POLE  AND  TOWER  LINES 


Percentage  of  elongation, 


1,400,000 


Bending   test,  180°  to    a 


ultimate  strength' 

diameter  equal  to  thickness  of  piece  tested,  without  fracture  on  out- 
side of  bent  portion. 

For  material  less  than  £{6  in.  and  more  than  y±  in.  in  thickness,  the 
folio  wing -modifications  shall  be  made  in  the  requirements  for  elongation: 

For  each  decrease  of  ^6  in.  in  thickness  below  %6  in.,  a  deduction 
of  2%  per  cent,  shall  be  made  from  the  specified  elongation. 

In  rounds'of  %  in.  or  less  in  diameter,  the  elongation  shall  be  measured 
in  a  length  equal  to  eight  times  the  diameter  of  section  tested. 

The  variation  in  cross-section  or  weight  of  more  than  23^  per  cent, 
from  that  specified  will  be  sufficient  cause  for  rejection,  except  in  the 
case  of  sheared  plates,  which  will  be  covered  by  certain  permissible 
variations. 

Rivets  and  Bolts. — A  well-made  hot-driven  rivet  is  superior 
to  even  a  turned  bolt  because  the  excess  length  of  the  rivet  has 
been  upset  into  contact  with  the  sides  and  irregularities  of  the 
hole,  and  because  in  cooling  the  contraction  of  the  rivet  presses 
the  riveted  pieces  together  and  develops  a  friction  which  increases- 
the  strength  of  the  joint.  The  use  of  turned  or  machined  bolts 
in  drilled  or  accurately  reamed  holes  would  be  of  prohibitive 
cost  for  pole  or  tower  work,  while  drawn-wire  bolts  (which  are 
truly  circular),  in  holes  enlarged  with  a  conical  reamer,  are  not 
worth  the  additional  expense.  Since  the  number  of  bolts  per 
joint  is  small,  rarely  over  two  and  frequently  but  one,  it  is 
probable  that  the  friction  between  the  connected  pieces  is 
negligible. 

For  transmission  line  construction  the  strength  of  a  rivet  or 
bolt  is  either  its  shearing  or  its  bearing  value.  The  former  is  the 
shearing  strength  per  square  inch  of  the  rivet  or  bolt  material 
multiplied  by  the  area  of  the  cross-section,  and  assuming,  as  will 
usually  be  the  case,  that  48, 000-58, 000-lb.  material  is  used,  we 
have  the  following  shearing  values: 

TABLE  21. — SHEARING  VALUES  OF  RIVETS  AND  BOLTS 


Working  values  (in  Ib.) 

Diameter  rivet  or 
bolt  (in  inches) 

Ultimate  shear  (in 
pounds)  (36,000  Ib. 
per  square  inch) 

Shop  rivets  (15,000 
Ib.  per  square  inch) 

Field  rivets  or  bolts 
(1  2,000  Ib.  per  square 
inch) 

H 

7,000 

2,900 

2,400 

M 

11,000 

4,600 

3,700 

y± 

16,000 

6,600 

5,300 

Vs 

21,500 

9,000 

7,200 

STEEL  POLES  AND  TOWERS 


105 


The  bearing  value  of  a  rivet  or  bolt  is  that  of  the  effective  area 
of  metal  pressed  together  during  the  transfer  of  stress,  and, 
therefore,  equals  the  product  of  the  diameter  of  the  rivet  or  bolt, 
the  thickness  of  the  thinner  riveted  piece,  and  the  unit  bearing 
value  of  the  material. 

TABLE  22. — ULTIMATE  BEARING 

Ultimate  bearing  (72,000  Ib.  per  square  inch) 


Diameter  rivet  or 

Thickness  of  thinner  connected  piece   (in  inches') 

H 

Me 

N 

Me 

M 

Me 

H 

4,500 

6,700 

9,000 

11,300 

13,500 

15,700 

H 

5,600 

8,400 

11,200 

14,000 

16,800 

19,600 

X 

6,700 

10,100 

13,500 

16,800 

20,200 

23,600 

% 

7,900 

11,800 

15,800 

19,700 

23,600 

27,600 

TABLE  23. — BEARING  VALUES  OF  RIVETS  AND  BOLTS' 

Working  values 


Diameter 
rivet  or 

Shop  rivets  (30,000  Ib.  per  square  inch) 

Field    rivets    or    bolts    (24,000    Ib. 
per  square  inch) 

inches) 

Thickness  of  thinner  connected  piece  (in  inches) 

tt 

Me 

M 

Me 

H 

H.     ||    H 

Me 

^ 

Me|     H 

Me 

H 

1,800 

2,800 

3,700 

4,700 

5,600 

6,500 

1,500 

2,200 

3,000 

3,700 

4,500 

5,300 

N 

2,300 

3,500 

4,700 

5,900 

7,000 

8,200 

1,800 

2,800 

3,700 

4,700 

5,600 

6,500 

H 

2,800 

4,200 

5,600 

7,000 

8,400 

9,800 

12,200 

3,300 

4,500 

5,600 

6,700 

7,900 

% 

3,200 

4,900 

6,500 

8,200 

9,800 

11,500 

2,600 

3,900 

5,300 

6,600 

7,900 

9,200 

In  bridge  and  building  construction,  it  is  customary  to  specify 
that  the  distance  between  rivet  holes  shall  be  not  less  than  three 
times  the  diameter  of  the  rivet,  and  the  distance  from  the  center 
of  the  hole  to  the  end  or  edge  of  the  piece  shall  be  not  less  than 
one  and  one-half  times  the  diameter  of  the  rivet.  In  practice, 
however,  particularly  when  using  small  thin  sections,  these 
minimum  distances  are  often  reduced.  In  transmission  line 
structures,  the  stresses  to  be  transferred  from  the  bracing  to  the 
main  members  are  usually  small  and  there  is  little  danger  of  the 
material  failing  between  or  outside  the  holes,  provided  an  excess- 

1  In  the  working  values  for  M-m-  material,  a  more  conservative  factor  of 
safety  has  been  used,  since  the  theoretical  ultimate  values  are  not  always 
obtained  in  practice.  For  instance,  it  has  been  found  by  test  that  ^-in. 
material  may  crumple,  allowing  the  bolt  to  pull  through  the  hole  at  a 
stress  less  than  the  theoretical  ultimate  bearing  value. 


106 


POLE  AND  TOWER  LINES 


ively  close  spacing  is  prohibited.  The  distance  from  a  hole  to  a 
rolled  edge  may  be  made  slightly  smaller  than  that  to  a  sheared 
edge  or  end,  since  the  material  of  the  former  is  free  from  any 
injury  due  to  the  shearing  process. 

In  Fig.  46  are  shown  the  minimum  spacing,  edge,  and  end 
distances,  for  each  size  of  rivet  or  bolt,  below  which  further 
reduction  is  inadvisable.  Where  clearance  will  permit,  the  end 
distances  shown  should  be  increased  about  in.  to  in.  From 


(  a  )     Minimum  Spacing  of  Rivets  &  Bolts 
( b  )  "         End  Distance     "  »• 

(  C  )  "          Edge  Distance    »  » 

FIG.  46. — Minimum  spacing,  edge,  and  end  distances. 

the  edge  distances  given  and  assuming  the  usual  thickness  of 
material  and  sizes  of  nut,  the  minimum  section  of  angle  to  be 
used  for  each  diameter  of  bolt  is  found  to  be : 

TABLE  24. — MINIMUM  ANGLE  SECTIONS 


Diameter  rivet   or  bolt 
(in  inches) 

Minimum  angle 

Yz 

IK  in.  L 

5/8 

1%  in.  L 

% 

234  m.  L 

% 

2Y2  in.  L 

Lacing. — The  function  of  lacing  is  to  stiffen  the  connected 
members  by  reducing  the  unsupported  length  of  the  compression 
section  and  also  to  transmit  shearing  stresses.  If  the  shear  is 
relatively  large,  the  limiting  condition  may  be  the  number  of 


STEEL  POLES  AND  TOWERS 


107 


rivets  connecting  the  lacing  to  the  main  section,  otherwise  it 
will  be  the  stiffness  of  the  lacing  itself.  That  is,  the  lacing  is  a 
compression  member  whose  strength  depends  on  its  ratio  of 
stiffness,  or  l/r.  Since  the  minimum  radius  of  gyration  of  a 
flat  or  bar  is  much  smaller  than  that  of  an  angle,  the  unsupported 
length  of  the  former  must  be  less.  Again,  flat  lacing  is  more  sub- 
ject to  accidental  injury  than  angle  lacing  because  a  slight  bend 
in  the  direction  of  the  thickness  may  easily  occur  and  make 
the  theoretical  compressive  strength  negligible. 


D 


FIG.  47. — Single  flat  lacing. 


FIG.  48. — Double  flat  lacing. 


The  value  of  r  of  a  lacing  bar  is  approximately  0.3  of  its 
thickness;  therefore,  increased  stiffness  can  be  obtained  only 
by  additional  thickness  or  by  reducing  the  length. 

When  double  lacing  is  used,  some  reduction  in  effective  length 
may  be  assumed  because  of  the  connection  at  the  intersec- 
tion. With  flat  lacing,  however,  it  is  not  correct  to  assume 
that  the  effective  length  is  the  distance  from  the  end  hole  to 
the  intersection. 

The  inclination  or  angle  a  of  the  lacing  affects  both  the  length 
of  the  bars  and  their  stress.  The  compressive  stress  in  lacing  is: 


shear  S  X  sec.  a 


or 


108 


POLE  AND  TOWER  LINES 


C  =  1.155  S  for  a  =  30° 
C  =  1.414  S  for  a  =  45° 
C  =  2.000  S  for  a  =  60° 

Therefore,  both  th£  length  of  the  bar  and  the  stress  will  be  in- 
creased 73  per  cent,  by  increasing  the  angle  a  from  30°  to  60°. 
The  reduced  strength  and  increased  stress  may  cause  either  the 
thickness  of  the  bar  or  the  strength  of  the  rivet  to  become  the 
limiting  condition. 

In  addition,  it  is  unwise  to  use  values  of  a.  much  in  excess  of 
30°  for  single  bars,  45°  for  double  bars,  and  45°  for  angles,  as 
the  stiffening  effect  of  a  light  member  connected  by  one  rivet 
is  very  small  with  excessive  inclinations. 

Angle  Lacing. — Owing  to  the  fact 
that  the  radius  of  gyration  of  an  angle 
is  larger  than  that  of  a  flat,  the  for- 
mer section  allows  a  considerable  in- 
crease in  the  width  of  the  main  mem- 
bers with  less  material  in  the  lacing. 
The  angle  section  may  depend  on 
the  size  of  the  bolt  needed  to  transmit 
the  stress  in  the  lacing  or,  if  the  latter 
is  turned  in,  on  the  permissible  end 
and  edge  distances. 

In  Table  25  will  be  found  the  values 
of  r  for  various  sections,  and  the  length 
corresponding  to  different  ratios  of  l/r. 
The  maximum  permissible  value  of  l/r  will  depend  to  some  extent 
on  the  character  of  the  service  'expected  of  the  bracing,  as  well  as 
on  its  position  in  the  structure.  Bracing  for  secondary  members, 
which  are  not  liable  to  accidental  injury  or  torsion,  may  be 
allowed  larger  ratios  than  main  compression  members,  which 
from  their  position  may  be  subject  to  both  injury  and  torsion. 
Again,  in  selecting  angle  sections  and  the  ratio  l/r  for  any  member, 
some  consideration  should  be  given  to  its  position  and  protective 
coating,  as  the  effect  of  these  may  make  it  advisable  to  employ  a 
thicker  angle.  Thus,  a  bracing  angle  with  the  outstanding  flange 
turned  in  and  upward,  and  the  angle  itself  in  a  vertical  or  in- 
clined plane,  is  less  subject  to  injury  either  from  accident  or 
corrosion  than  a  similar  angle  reversed  or  in  a  horizontal  plane. 


FIG.  49. — Single  angle  lacing. 


STEEL  POLES  AND  TOWERS 
TABLE  25. — ANGLES 


109 


Section 

Area 

Weight 

Least 

r 

Length    in    inches    corresponding    to     various 
values  of  //r 

60 

80 

100 

120 

150 

180 

220 

l^XlHXfia 

0.48 

1.6 

0.26 

x  y\ 

0.63 

2.1 

0.26 

1WX1V4XH 

0.36 

1.2 

0.30 

18 

27 

30 

36 

45 

54 

66 

XMo 

0.53 

1.8 

0.29 

17 

23 

29 

35 

44 

52 

64 

xu 

0.69 

2.3 

0.29 

17 

23 

29 

35 

44 

52 

64 

154X1J4XH 

0.36 

1.2 

0.27 

XMo 

0.53 

1.8 

0.27 

XH 

0.69 

2.3 

0.27 

1?4X1%XH 

0.42 

1.4 

0.35 

21 

28 

35 

42 

52 

63 

77 

XMo 

0.63 

2.1 

0.34 

20 

27 

34 

41 

51 

61 

75 

XH 

0.82 

2.8 

0.34 

20 

27 

34 

41 

51 

61 

75 

2     XlHXMe 

0.57 

2.0 

0.27 

17 

23 

29 

35 

44 

52 

64 

XH 

0.75 

2-.  6 

0.27 

17 

23 

29 

35 

44 

52 

64 

2     X2     X*io 

0.72 

2.4 

0.39 

23 

31 

39 

47 

58 

70 

86 

XH 

0.94 

3.2 

0.39 

•23 

31 

39 

47 

58 

70 

86 

2MX2J4XM* 

0.81 

2.8 

0.44 

26 

35 

44 

53 

66 

79 

97 

XH 

1.07 

3.6 

0.44 

26 

35 

44 

53 

66 

79 

97 

2V$Xl^X?i6 

0.72 

2.4 

0.33 

20 

26 

33 

40 

49 

59 

73 

XH 

0.94 

3.2 

0-32 

19 

26 

32 

38 

48 

58 

70 

2^X2     X^e 

0.81 

2.8 

0.43 

26 

34 

43 

52 

64 

77 

95 

XJ4 

1.07 

3.6 

0.42 

25 

34 

42 

50 

63 

76 

92 

2HX2J4XM6 

0.91 

3.1 

0.49 

29 

39 

49 

59 

73 

88 

108 

XW 

.19 

4.1 

0.49 

29 

39 

49 

59 

73 

88 

108 

XSid 

.47 

5.0 

0.49 

29 

39 

49 

59 

73 

88 

108 

2HX2HX91* 

.00 

3.4 

0.54 

32 

43 

54 

65 

81 

97 

119 

XH 

.32 

4.5 

0.54 

32 

43 

54 

65 

81 

97 

119 

x^« 

.63 

5.6 

0.54 

32 

43 

54 

65 

81 

97 

119 

3     X2     X3io 

.91 

3.1 

0.44 

26 

35 

44 

53 

66 

79 

97 

XN 

.19 

4.1 

0.43 

26 

34 

43 

52 

64 

77 

95 

Xtf* 

.47 

5.0 

0.43 

26 

34 

43 

52 

64 

77 

95 

3     X2^X^6 

.00 

3.4 

0.53 

32 

42 

53 

64 

79 

95 

117 

X!4 

.32 

4.5 

0.53 

32 

42 

53 

64 

79 

95 

117 

X51o 

.63 

5.6 

0.53 

32 

42 

53 

64 

79 

95 

117 

3     X3     XMe 

.09 

3.7 

0.59 

35 

47 

59 

71 

88 

106 

130 

XH 

.44 

4.9 

0.59 

35 

47 

59 

71 

88 

106 

130 

XMe 

.78 

6.1 

0.59 

35 

47 

59 

71 

88 

106 

130 

3KX2HXH 

.44 

4.9 

0.54 

32 

43 

54 

65 

81 

97 

119 

XMo 

.78 

6.1 

0.54 

32 

43 

54 

65 

81 

97 

119 

3^X3     XW 

.56 

5.4 

0.63 

38 

50 

63 

76 

94 

113 

139 

XM« 

.94 

6.6 

0.63 

38 

50 

63 

76 

94 

113 

139 

3HX3HXM« 

2.09 

7.2 

0.69 

41 

55 

69 

83 

103 

124 

152 

XH 

2.49 

8.5 

0.68 

41 

54 

68 

82 

102 

122 

150 

XMo 

2.88 

9.8 

0.68 

41 

54 

68 

82 

102 

122 

150 

XMi 

3.25 

11.1 

0.68 

41 

54 

68 

82 

102 

122 

150 

4     X4'    Xff« 

2.41 

8.2 

0.79 

47 

63 

79 

95 

118 

142 

174 

XX 

2.86 

9.8 

0.79 

47 

63 

79 

95 

118 

142 

174 

XMe 

3.31 

11.3 

0.78 

47 

62 

78 

94 

117 

140 

172 

X^ 

3.75 

12.8 

0.78 

47 

62 

78 

94 

117 

140 

172 

5     X5     X9i 

3.61 

12.3 

0.99 

59 

79 

99 

119 

148 

178 

218 

XJii 

4.19 

14.3 

0.98 

59 

78 

98 

118 

147 

176 

216 

X*4 

4.75 

16.2 

0.98 

59 

78 

98  . 

118 

147 

176 

216 

6     X6     XH 

4.36 

14.9 

1.19 

71 

95 

119 

143 

178 

214 

262 

XMfl 

5.06 

17.2 

1.19 

71 

95 

119 

143 

178 

214 

262 

X^ 

5.75 

19.6 

1.18 

71 

94 

118 

142 

177 

212 

260 

XHe 

6.44 

21.9 

1.18 

71 

94 

118 

142 

177 

212 

260 

110 


POLE  AND  TOWER  LINES 


Tower  Connections. — As  it  is  usually  advantageous,  from  a 
manufacturing  standpoint,  to  maintain  like  punching  on  both 
flanges  of  the  main  leg  angles,  the  panel  points  are  often  at  the 
same  elevation  on  all  four  sides  of  the  tower  and  the  holes  are 

opposite  each  other.  This 
necessitates  clipping  the  out- 
standing flange  of  one  brace  at 
each  panel  point,  to  clear  the 
inside  brace  on  the  adjoining 
face  and  also  to  provide  space 
for  the  insertion  of  the  inside 
connection  bolt  (Fig.  50). 

An  alternative  method  is  to 
stagger  the  main  panel  points  a 
few  inches  and  thus  obtain  the 
necessary  clearances  without 
clipping.  To  do  this,  and  also 

maintain  like  bracing  angles  on  all  faces  of  the  tower,  it  is  neces- 
sary to  make  the  tower  out  of  square,  so  that  the  increased 
width  of  two  opposite  faces  will  compensate  for  the  greater 
length  of  the  diagonal  bracing  in  its  lowered  position. 


FIG.  50. — Bracing  connection, 


FIG.  51. — Bracing  connection. 

The  outstanding  flanges  of  horizontal  or  inclined  angles  should 
always  be  turned  up,  as  in  this  position  they  drain  and  dry  quickly 
and  do  not  collect  dirt  or  hold  water.  For  similar  reasons 
it  is  inadvisable  to  use  any  closed  pockets,  or  semi-closed  pockets, 
anywhere  in  the  structure,  as  they  are  certain  to  become  clogged 


STEEL  POLES  AND  TOWERS 


111 


with  refuse  and  filled  with  water.  Since  moisture  is  a  necessary 
condition  of  all  decay  and  corrosion,  rapid  and  thorough  drainage 
are  essential  to  a  good  design  whether  the  material  be  timber  or 
steel. 


FIG.  52. — Suspension  insulator      FIG.  53. — Strain  insulator  connections, 
connections. 


FIG.  54. 


FIG.  55. 


Single  3^-in.  bolt  connections  should  be  prohibited  in  the  main 
bracing  system  of  wide-base  towers,  except  possibly  for  the 
connection  of  secondary  members  such  as  sub-panel  struts,  whose 
sole  function  is  to  reduce  the  unsupported  length  of  other 


112 


POLE  AND  TOWER  LINES 


members.  Such  struts  have  frequently  been  given  odd  inclina- 
tions as  compared  with  the  main  diagonal  system,  with  a  result 
far  from  pleasing  in  appearance.  It  is  generally  true  that  the 
diagonal  bracing  on  tangent  towers  with  well-inclined  main  legs 
has  to  withstand  only  relatively  low  stresses.  In  the  frantic 
endeavor  to  reduce  costs,  this  fact  has  led  to  the  use  of  widely 
separated  bracing,  which  is  incapable  of  properly  supporting 
the  most  important  section — the  main  leg  members.  In  other 
cases,  in  an  effort  to  obtain  the  greatest  possible  theoretical 
strength,  the  bracing  has  been  closely  spaced,  but  cut  down  in 
section  to  such  members  as  \Y±  in.  X  IJi  in.  X  -Hj-in.  angles 

with  large  values  of  — 


FIG.  56. 

The  connection  of  disc-type  insulators  to  steel  poles  or  towers 
is  usually  made  by  providing  a  U-bolt  or  plate  into  which  a  hook 
is  inserted.  In  other  cases,  the  connection  hole  is  made  in  the 
crossarm  angles  as  shown  in  Fig.  54. 

There  is  no  particular  merit  in  one  form  of  connection  rather 
than  in  another,  provided  the  thickness  of  the  crossarm  material 
is  sufficient  to  transmit  the  stress  to  the  arms.  The  thickness  of 
the  material  outside  the  hook  hole  must  be  about  %  in.  on  account 
of  the  spread  of  the  hook  and  the  inside  edges  of  the  hole  should 


STEEL  POLES  AND  TOWERS 


113 


preferably  be  rounded,  otherwise  the  hook  will  bear  upon  two 
points  only. 

Latticed  Poles. — Square,  latticed,  structural-steel  poles  may  be 
of  any  width  from  that  of  true  narrow-base  poles  used  along 
curb  lines  to  the  wide  poles  which  are  in  reality  towers.  There  is 
no  fixed  dividing  line  between  a  pole  and  a  tower,  unless  it  be 
that  of  strength  and  rigidity,  or  possibly  the  use  of  widths  which 
preclude  shop  riveting  and  shipment  assembled.  By  far  the 


FIG.  57. — Steel  crossing  pole. 

greater  number  of  the  structural-steel  poles  used  are  square  in 
cross-section,  one  angle  at  each  corner,  and  assembled  and 
riveted  before  shipment.  In  the  case  of  long  poles,  it  will  fre- 
quently be  found  advantageous  to  ship  in  two  sections,  and  bolt 
them  together  in  the  field.  There  is  no  reasonable  objection  to 
the  use  of  such  field  bolts,  provided  a  splice  is  used  of  sufficient 
strength  and  length.  The  splice  angle  can  be  made  an  interior 
splice,  with  the  root  of  the  angle  ground  to  fit  the  fillet  of  the 
main  legs,  and  thus  be  comparatively  unobtrusive  in  the  final 
appearance  of  the  pole. 

Several  types  of  poles  are  in  use,  the  most  common  being 


114 


POLE  AND  TOWER  LINES 


those  with  a  regular  bevel  or  those  with  parallel  legs.  Parabolic 
slopes  have  been  used  and  they  present  a  very  graceful  appear- 
ance under  favorable  conditions,  although  the  rapid  increase  in 
width  for  longer  poles  may  result  in  an  inconvenient  spread  at  the 
ground  line.  The  parabolic  slope  has  its  true  function  in  the 
application  to  very  high  towers  of  uniform  height. 

In  some  cases,  the  top  portion  of  regularly  sloped  poles  has 
been  made  with  parallel  sides  in  order  to  maintain  like  punch- 


FIG.  58. — Railroad  crossing  pole,  13,000  volt  wires. 


ing,  length  and  pin  spacing  of  the  crossarms.  This  does  not 
always  give  a  good  appearance,  however,  on  account  of  the 
marked  bend  at  the  lowest  arm. 

The  foundation  or  anchorage  of  latticed  poles  may  be  made  in 
three  general  ways,  i.e. :  by  simply  burying  the  lower  portion,  by 
providing  separate  ground-stub  angles  as  in  a  rigid  tower,  or 
by  a  base  plate  or  plates  attached  to  anchor  bolts.  Further, 


STEEL  POLES  AND  TOWERS 


115 


the  material  entering  the  ground   may  be  either  painted  or 
galvanized,  although  the  former  should  be  encased  in  concrete. 

Base  plates  and  anchor  bolts  are  sometimes  more  expensive  in 
material  and  workmanship  than  either  of  the  other  designs,  but 
allow  concrete  foundations  to  be  built  in  advance  with  the 
least  probability  of  error  in  setting  the  connections  to  the  super- 
structure. 


FIG.  59. — Arrester  house  and  guyed  terminal  pole. 

As  shown  in  Fig.  59,  some  quite  elaborate  bases  have  been  used, 
the  general  purpose  of  these  being  to  more  firmly  fix  the  base 
of  the  column  and  to  provide  an  excess  of  material  against  cor- 
rosion. The  real  benefit  derived  from  such  construction, 
however,  is  open  to  question,  and  the  gain  does  not  seem  com- 
mensurate with  the  cost.  Steel  poles  can  readily  be  protected 
against  injury  from  accidental  collisions,  overflow,  high  water, 


116 


POLE  AND  TOWER  LINES 


FIG.  60. — Two-circuit  steel  poles. 


FIG.  61. — Steel  poles  with  wooden  arms,  11,000  volts. 


STEEL  POLES  AND  TOWERS 


117 


etc.,  by  the  simple  expedient  of  encasing  the  lower  portion  in 
concrete.  Either  a  section  of  the  latticed  pole  itself  may  be 
encased  or  long  stub  angles,  embedded  in  high  foundations, 
may  be  attached  to  the  superstructure  in  the  usual  mariner 
(Fig.  60). 

Figs.   63   and   64  are  views   of    the   north   and    east   sides 
respectively  of  a  square  latticed  structural  steel  pole  tested  to 


FIG.  62. 

destruction.     The  views  are  unusually  good  in  that  they  show 
very  clearly  the  typical  compression  failure  by  buckling. 

It  will  also  be  apparent  that  angle  lacing  is  very  effective  in 
preventing  buckling  in  the  plane  of  the  lacing,  but  that  it  has 
only  a  little  restraining  influence  at  right  angles  to  its  plane.  It 
is  further  apparent  that  with  latticed  steel  poles  failure  does  not 
necessarily  cause  the  pole  to  fall  or  break  off.  Instead  the 


118 


POLE  AND  TOWER  LINES 


pole  buckles  near  the  base  allowing  the  top  to  deflect,  thus  de- 
creasing the  wire  tension  but  without  causing  the  wires  to  fall  to 
the  ground. 

The  concrete  foundations  shown  are  relatively  large  for  the 
poles  they  support,  but  were  considered  desirable  in  view  of 
the  location  in  a  river  bank. 

The  design  of  square  latticed  poles,  in  general,  may  be  resolved 
into  a  determination  of  the  stresses  at  the  ground  line  or  rather 
in  the  first  panel  above  ground.  This  statement  is  based  on 


FIG.  63.  FIG.  %  64. 

Latticed  steel  pole,  test. 


the  assumption  that,  owing  to  the  tops  being  relatively  wider 
than  in  wood  poles,  the  upper  portion  of  the  pole  has  an  excess 
width  as  compared  with  the  lowest  panel.  It  is  further  predi- 
cated on  there  being  no  attempt  made  to  seriously  reduce  the 
sections  of  the  material  in  the  upper  half.  In  the  case  of  para- 
bolic slopes,  stress  determinations  must  be  made  at  various 
heights  since  the  widths  presumably  follow,  more  or  less  closely, 
the  changes  in  bending  moment,  so  the  weakest  section  may 
be  anywhere.  Owing  to  the  greater  rigidity  of  pole  frames, 


STEEL  POLES  AND  TOWERS 


119 


the  breaking  strength  per  unit  of  area  in  a  pole  will  exceed 
that  in  a  wide-base  tower.  Again,  since  the  main  legs  have 
little  inclination,  the  web  system  is  compelled  to  carry  the 
shearing  stresses,  which  in  a  tower  are  partly  carried  by  the 
main  legs.  For  these  reasons,  the  web  or  lattice  is  more  often 
limited  by  the  strength  required 
than  is  the  bracing  of  a  wide-base 
tower.  Shearing  stresses  must, 
therefore,  be  computed,  and  the 
lattice  and  its  connection  to  the 
main  legs  designed  accordingly. 

Single  flat  lacing  should  not  be 
used,  except  for  small  stresses  and 
in  narrow  widths,  since,  as  stated 
before,  its  strength  is  low  and  it 
is  easily  injured.  Double  flat  lac- 
ing is  applicable  to  greater  stresses 
and  widths,  but  it  is  often  not  as 
economical  as  angle  lacing.  In 
any  case  the  strength  of  the  pole 
depends  on  the  unit  strength  of 
the  weakest  unsupported  length, 
which  is  usually  the  lowest  panel 
but  may  be  the  entire  pole  if  the 
width  is  small  and  the  height  great. 
That  is,  the  l/r  of  the  entire  cross- 
section  of  the  pole  may  be  greater 
than  that  of  any  single  panel.  The 
character  and  spacing  of  the  lattice 
will  determine  to  a  large  extent 
the  amount  of  support  afforded  by 
it  to  the  main  leg  angles  at  the 
panel  points. 

When  both  faces  of  a  pole  are 
connected  with  lacing  at  the  same 

level,  the  unsupported  length  of  the  main  leg  is  the  distance 
between  panel  points.  If,  however,  the  lacing  is  staggered, 
so  that  the  support  is  in  one  direction  only  at  each  panel 
point,  the  unsupported  length  of  the  main  leg  is  somewhere 
between  a  half  and  a  whole  panel  length. 


FIG.  65. 


120 


POLE  AND  TOWER  LINES 


DESIGN  OF  STEEL  POLE 

One  %-in.  Siemens-Martin  galvanized  stranded-steel  ground  wire. 

Three  No.  1  hard-drawn  stranded  copper,  33,000  volts. 

Span         =  400  ft. 

Nor.  sag   =  9  ft.  0  in.     Nor.  tension  (GOT.)  =  570  Ib. 

Max.  sag  =  10  ft.  6  in.    Max.  tension  (0°F.,  ^-in.  ice,  8  Ib.  wind)  =  1960  Ib. 


Ground  Wire 

- 


70 


v_ 


Clearance 


} 


T    k™2'2? 


—  36 


J  c.g. 


FIG.  66. — Design  of  steel  pole. 

Elastic  limit,  No.  1  wire  =  2180  Ib. 

Wind  pressure  on  wires  (^-in.  ice,  8  Ib.  per  square  foot  wind) : 

%-iri.  ground  wire  =  0.917  X  400  =  367  Ib. 

No.  1  conductor      =  0.885  X  400  =  354  Ib. 

Wind  on  pole  =  13  Ib.  per  square  foot  X  1^  times  exposed  area 

of  windward  side  =  20  Ib.  per  lineal  foot. 


STEEL  POLES  'AND  TOWERS  121 

With  the  above  transverse  loading  and  no  broken  wires,  the  compressive 
stress  in  the  leg  in  the  lowest  full  panel  above  the  foundation  is  ob- 
tained by  taking  moments  of  the  forces  about  the  panel  point  2  ft.  1  in. 
above  the  foundation. 

Ground  wire          367  Ib.  X  42.4  =  15,560  ft.-lb. 
Power  wires  354  Ib.  X  39.9  =  14,120  ft.-lb. 

Power  wires  354  Ib.  X  2  X  37.4  =  26,480  ft.-lb. 

42  42 
Wind  on  pole  =  20  Ib.  X =  17,980  ft.-lb. 

Total  bending  moment       =  74,140  ft.-lb. 

Since  the  lever  arm  of  the  resisting  forces  =  1.9  ft. : 

74,140  ft.-lb.  -r-  (1.9ft.  X  2  legs)  =  19,500  Ib. 

Vertical  load— steel  =  1700  Ib. 

Vertical  load — wires  and  insulators  =  1500 


3200  Ib.  -=-  4  legs  =       800  Ib. 
Total  compressive  stress  in  each  leg  =  20,300  Ib. 

Since  1  L  3  X  3  X  M  =  1.44  sq.  in.,  the  maximum  unit  stress  in  each 
leg  =  20,300  -5-  1.44  =  14,100  Ib.  per  square  inch. 

If  the  side  face  of  the  pole  is  the  same  as  the  view  shown,  the 
leg  is  restrained  from  buckling  in  one  direction  by  the  intersect- 
ing diagonals,  so  the  maximum  l/r  will  be  either  the  full  panel 
length  divided  by  the  radius  of  gyration  parallel  to  the  leg,  or 
the  half  panel  length  divided  by  the  least  radius  of  gyration,1  i.e., 

I       44  in.  I        22  in. 


The  ultimate  strength  of  the  angle  based  on  the  greater  value  of 
l/r  and  the  curve  in  Fig.  33  is  35,000  Ib.  per  square  foot;  there- 

35  000 
fore  the  factor  of  safety  is      '         =  2.5. 


Similarly  the  tensile  stress  in  the  other  leg  can  be  obtained  by 
taking  moments  about  the  lowest  panel  point  and  subtracting 
the  vertical  load  stress,  which  is  always  compression.  For 
tensile  stresses  the  area  of  the  angle  should  be  reduced  by  the 
area  of  one  rivet  hole. 

Since  the  ultimate  strength  in  tension  is  about  60,000  Ib.  per 
square  inch,  the  compressive  stress  is  generally  the  governing 
factor. 

Stress  in  Diagonals.  —  The  horizontal  shear  at  the  lowest  crossarm  is: 
xSee  page  119,  the  assumption  of  the  half  panel  length  is  conservative, 


122  POLE  AND  TOWER  LINES 

Ground  wire  =  367  Ib.  XI  =  367 
Conductors  =  354  Ib.  X  3  =  1062 
Wind  on  pole  =  20  Ib.  X  6  =  120 

Total  =  1549  Ib. 

This  will  be  carried  by  the  web  systems  of  two  faces,  making 
a  shear  of  775  Ib.  per  face.  With  inclined  legs,  the  stress  in  the 
diagonals  will  decrease  from  the  lowest  arm  to  the  ground.  Since 
the  taper  of  the  legs  in  this  case  is  small,  the  stress  in  the  diagonal 
just  below  the  arm  can  be  said,  with  only  a  small  error,  to  equal 
the  shear  multiplied  by  the  secant  of  the  slope. 

Assuming  a  45°  slope  for  the  diagonals,  the  stress  will  be  775 
Ib.  X  1.414  =  1100  Ib. 

Stress  in  Crossarms. — The  crossarms  should  be  designed  for  either  the 
maximum  ice  and  wind  loads  on  both  spans  or  for  the  maximum  ice  and 
wind  loads  on  one  span  combined  with  a  longitudinal  load  due  to  the  break- 
ing of  the  wire  in  the  other  span. 

CONDITION  No.  1 

Vertical  load  (*^-in.  ice  on  wires)  —  0.770  Ib.  per  foot  X 

400ft =         308  Ib. 

Insulator  and  pin =  25  Ib. 

333  Ib. 

333  Ib.  X  35  in. . .  =  11,680  in.-lb. 

3  I2 
Weight  of  arm  =  15  Ib.  per  foot  X    ^  X  12   =        860  in.-lb. 

Horizontal  load  (8  Ib.  per  square  foot  wind  on  wire  - 
0.885  Ib.  per  foot  X  400  ft.)  = 
354  Ib.  X  12  in =  4,250  in.-lb. 

Total  bending  moment =    16,770  in.-lb. 

2  Is   3>£  in.   X  3  in.  X  Ke  in.  -  =  2  X  0.95  =  1.90. 

c 

Max.  unit  stress  in  crossarm  =  16,770  -*-  1.90  =  8800  Ib.  per  square  inch. 

CONDITION  No.  2 

Vertical  load  (3^-in.  ice  on  wires)  —  0.770  Ib.  per  foot  X 

200ft =        154  Ib. 

Insulator  and  pin =          25  Ib. 

179  Ib. 


STEEL  POLES  AND  TOWERS 


179  lb.  X35in. 


3.12 


Weight  of  arm  =  15  lb.  per  foot  X  ~2~  X  12 

Horizontal  load  (8  lb.  per  square  foot  wind  on  wire  — 
0.885  lb.  per  foot  X  200  ft.)  = 
177  lb.  X  12  in.. 


123 

6270  in.-lb. 
860  in.-lb. 


2120  in.-lb. 


Total  bending  moment =      9250  in.-lb. 

9250  -4-  1.90  =  4900  lb.  per  square  inch. 
Longitudinal  load: 

1960  lb.  X  -J^RQ  =  3720  lb.  -=-  1.94  sq.  in.  =  1900  lb.  per  square  inch. 
Maximum  unit  stress  in  crossarm =  6800  lb.  per  square  inch. 

It  should  be  noted  that  comparatively  few  insulators  and  pins 
can  safely  carry  the  longitudinal  load  assumed  in  Condition  No. 
2,  even  if  the  tie  wires  were  able  to  transmit  the  load  to  the 
insulator.  Generally,  only  strain  poles  or  towers  are  able  to 


FIG.  67. — Curb-line  poles. 

fully  meet  this  condition,  as  they  are  provided  with  either  strain 
insulators  or  double-pin  insulators  and  more  effective  tie  or 
clamping  devices. 

Curb-line  Poles. — Where  high-voltage  lines  are  located  along 
curbs,  it  is  important  that  the  construction  be  of  a  high  degree 
of  excellence,  and  that  structures  be  used  which  combine  strength, 


124  POLE  AND  TOWER  LINES 

a  restricted  width,  and  at  least  some  esthetic  qualities.  A  rela- 
tively high  amount  of  insulation,  with  the  consequent  freedom 
from  electrical  failure,  affords  the  greatest  protection  for  the 
least  expenditure.  The  width  of  poles  must  necessarily  be 
restricted  at  the  ground  line,  the  maximum  permissible  width 
being  from  24  in.  to  about  28  in.  These  dimensions  are  not  fixed 
by  any  definite  rule,  but  result  from  the  precedent  established 
by  the  use  of  large  wooden  poles.  There  are  many  places, 
however,  where  greater  widths  would  not  create  any  real  ob- 
struction nor  presumably  any  active  criticism.  There  should, 
in  fact,  be  less  objection  to  a  line  of  well-designed  steel  poles  than 
to  wood  poles,  since  their  appearance  is  superior  and  only  about 
'one-half  as  many  poles  are  required. 

The  surfaces  of  the  poles  which  may  possibly  come  into  contact 
with  pedestrians  should  be  free  from  projecting  edges;  therefore, 
the  latticing  should  be  inside  the  main  legs  and  its  connections 
riveted  rather  than  bolted. 

It  will  sometimes  be  found  advantageous  to  prevent  the  climb- 
ing of  poles  by  unauthorized  persons.  This  can  be  done  by  clamp- 
ing wire  netting  against  the  lacing  a  short  distance  above  the 
ground,  or  by  filling  the  interior  of  the  pole  with  concrete.  The 
latter  is  not  expensive,  the  forms  being  extremely  simple,  and  it 
strengthens  the  pole  both  for  general  use- and  as  a  hub  guard. 

Triangular  Poles. — The  three-legged  poles  used  heretofore 
have  generally  been  of  a  proprietary  type  employing  U-shaped 
main  legs  fastened  at  intervals  with  horizontal  cast  spreaders, 
but  a  few  have  been  built  of  structural  angles. 

The  material  of  the  former  is  usually  rerolled  rail  of  greater 
unit  strength  but  much  harder  and  more  brittle  than  structural 
steel.  Owing  to  the  shape  and  small  flanges  of  U  sections,  as 
well  as  the  hardness  of  the  material  composing  them,  it  is  not 
practicable  to  lattice  the  main  legs  by  a  true  web  system.  In 
poles  of  this  type  the  main  legs  are  inclined  more  than  is  usual 
in  square  latticed  steel  poles,  and  the  shearing  stresses  must  be 
carried  by  the  main  leg  sections. 

In  any  structure  having  a  triangular  cross-section,  the  strength 
is  not  the  same  in  all  directions;  therefore,  three-legged  poles  are 
not  well  adapted  by  their  form  to  withstand  heavy  loads. 

When  built  of  homogeneous  material,  which  is  difficult  to  in- 
sure in  rerolled  stock,  such  poles  deflect  considerably  and  will 
bend  without  actual  fracture  much  more  than  square  latticed 


STEEL  POLES  AND  TOWERS 


125 


poles.  Failure  of  three-legged  poles  should,  therefore,  be  con- 
sidered as  occurring  when  a  permanent  bend  is  produced,  or 
when  the  fastenings  become  loosened. 

The  logical  service  for  poles  of  this  design  has  been  demon- 
strated by  practice  to  be  for  supporting  light  lines  in  locations 


65  Ft.  High 
110000  Volts 


64  Ft.  High 
110000  Volts 


43  Ft.  High 
102000  Volts 


62.5  Ft.  High 
70000  Volts 


/\/\ 


60  Ft.  High 
60000  Volts 


60  Ft.  High 
50000  Volts 


60  Ft.  High 
45000  Volts 


60  Ft.  High 
44000  Volts 


FIG.  68. — Types  of  towers 


involving  difficult  transportation,  the  poles  being  "  knocked 
down,"  shipped  in  light-weight  packages,  and  assembled  in  the 
field. 

Wide -base  Towers. — The  forms  of  the  frames  which  have 
been  used  in  wide-base  towers  are  of  many  types,  as  shown  by 
Fig.  68a,  6,  c,  d,  e,  f,  g  and  h.  The  majority  of  designs  are 


126 


POLE  AND  TOWER  LINES 


determinate  frames,  i.e.,  those  in  which  the  stresses  may  be 
computed  directly  and  definitely.  Some,  however,  are  what 
are  termed  indeterminate  frames,  since  all  the  stresses  cannot 
be  computed  directly  owing  to  the  fact  that  there  are  several 
paths  through  which  the  loads  may  be  carried  to  the  ground. 

In  such  designs  the  actual  distribution  of  stress  will  depend  in 
part  on  the  relative  rigidity  of  the  different  paths.  Although 
it  is  entirely  possible  to  build  indeterminate  frames  having  any 
necessary  strength,  the  practice  involves  a  liability  of  error. 

In  general,  the  designs  show  a  direct  transfer  of  the  tension 
and  compression  elements  of  the  bending  moment  through  the 
main  legs,  and  a  more  or  less  complicated  stiffening  system  whose 
chief  function,  in  some  cases,  is  to  provide  local  support  for  the 
main  legs. 

The  actual  sections  used  for  various  members,  even  for  some- 
what similar  installations,  present  marked  variations.  In  fact 
there  is  no  known  structural  theory  which  would  make  some  de- 
signs desirable  from  either  the  purchaser's  or  the  manufacturer's 
standpoint.  For  instance,  a  certain  installation  has  1  L  4"  X 
4"  X  M"  f°r  the  main  legs,  the  panel  length  being  13  ft. 
The  corresponding  l/r  is  therefore  202,  which  is  excessive  for  a 
main  compression  member. 

In  order  to  show  more  clearly  the  relative  undesirability  of  the 
section  in  question,  it  may  be  compared  with  two  other  sizes  of 
angle  as  follows: 


Area 

Length 

l/r 

Breaking 
strength  per 
square   inch 

Total  breaking 
strength 

1L  4in.X4in.X%  in. 
1L  5in.X5in.X%6  in. 
1L  6in.X6in.XK6  in. 

5.44 
5.31 
5.06 

13ft. 
13ft. 
13ft. 

202 
159 
131 

12,000  Ib. 
16,000  Ib. 
20,000  Ib. 

65,200  Ib. 
85,000  Ib. 
101,200  Ib. 

It  is  evident  from  the  foregoing  that  an  L  6"  X  6"  X  KG"? 
having  a  much  smaller  value  of  l/r,  would  have  been  stiffer, 
stronger,  lighter,  and  more  readily  fabricated  than  the  section 
used.  In  fact,  either  the  5"  X  5"  or  the  6"  X  6"  angle  would 
have  been  a  stronger  and  cheaper  section. 

It  may  be  observed  further  that  the  actual  factor  of  safety 
of  the  construction  in  question,  under  the  maximum  load  assumed 
in  its  design,  is  a  minus  quantity.  This  is  due  solely  to  the  ex- 
cessive value  'of  l/r  or,  in  other  words,  inadequate  bracing. 


STEEL  POLES  AND  TOWERS 


127 


FIG.  69. 


FIG.  70. 


FIG.  71. 


TABLE  27.—  KEY  TO  TOWER  SECTIONS.     Figs.  69,  70,  71. 

Mark  Section 

1  ................  :  .............  L  \Y2  X  IY2  X  H 

2  ..............................  L1%X  m  X  YB 

3  ..............................  L2  X  \1A  X  M 

4  ..............................  L2  X2  X  K 

5  ............  , 

6 

7  ..............................  L2}£  X2  X 

8 
9 

10  .........................  .'....  L3  X3  X 


12 


X  3^  X 


13  ..............................    L4  X  4  X 


14 
15 
16 


L4  X  4  X  M  X  y1Q 
L4  X  4  X  K  X  H 
Chan.  4  in.—  5.25  Ib 


17  ..............................   Chan:  5  in.—  6.5  Ib. 

Flexible  Frames.  —  The  steel  A-frame  trolley-wire  support  and 
transmission  pole  shown  in  Fig.  72  is  typical  of  some  of  the 
lighter  installations  in  Europe,  where  the  A-frame  was  originated. 
This  view  is  of  additional  interest  in  that  the  light  brackets  to 
which  the  trolley-wire  messenger  cables  are  attached  would  have 


128 


POLE  AND  TOWER  LINES 


little  or  no  strength  to  withstand  a  broken  cable,  thus  tending  to 
show  that  failures  in  wires  are  not  very  seriously  considered  by 
the  European  designers  of  such  structures.  The  illustration 
also  shows  a  pair  of  grounding  arms  under  the  transmission  cir- 
cuits at  the  top  of  the  pole. 

TABLE  26. — RECORD  OF  SINGLE  AND  DOUBLE  CIRCUIT  WIDE  BASE  TOWERS. 
REPORTS  (1915)  PROM  SIXTEEN  LINES  HAVING  A  TOTAL  OP  7362  TOWERS 

IN  SERVICE 


Number 
of 
towers 

Tower 
failures 

Remarks 

244 

None 

A  crossarm  twisted,  due  to  burnt  conductor. 

378 

None 

1041 

None 

370 

None 

64 

None 

243 

None 

Three  conductors  burnt,  due  to  contacts  when  heavy 

sleet  unbalanced  sags  during  removal. 

1079 

None 

593 

None 

184 

None 

33 

None 

851 

None 

110 

None 

324 

None 

Four  broken  insulator  connections. 

913 

None 

A  number  of  crossarm   hanger    rods    have 

failed. 

Several  conductors  burnt. 

748 

1 

Due  to  guy  failure.     Also  about  70  breaks 

in  con- 

ductors.     (Storm    of    Apr.    2,     1915  —  worst    on 

record.) 

187 

1 

Compression  leg  of  tower  carrying    1250-ft 

.    span. 

Also  a  number  of  slight  buckles  in  other 

towers. 

7362 

(Storm  of  Apr.  2,  1915.) 

When  flexible  frames  were  first  used  in  this  country,  it  was 
customary  to  insert  a  rigid  or  dead-ending  tower  at  corners  and 
at  intervals  of  about  five  spans  on  tangents.  In  recent  years, 
however,  there  has  been  a  tendency  to  omit  some  of  these 
stiffening  structures  on  the  theory  that  there  was  no  real  danger 
of  the  line  falling  longitudinally  like  a  "house 'of  cards."  In 
view  of  a  number  of  accidents  that  have  occurred  on  such  lines, 
it  would  seem  desirable  either  to  return  to  the  former  practice  or 
to  obtain  the  effect  of  stiffening  structures  by  a  more  liberal  use 
of  guys. 

A  general  objection  to  the  flexible  pole  or  frame  is  not  in- 


STEEL  POLES  AND  TOWERS 


129 


tended,  as  they  may  have  a  proper  usefulness  in  the  construction 
of  the  lighter  and  less  important  lines.  It  is  further  probable 
that  some  criticisms  of  such  construction  would  be  more  accu- 
rately directed  to  their  shallow  foundations,  span  lengths,  details, 
and  incorrect  installation  than  to  their  use  under  favorable 
conditions. 

Too  much  emphasis  has  been  placed  upon  the  need  of  flexi- 
bility, and  to  spend  any  efforts  in  providing  greater  flexibility 
than  is  found  in  the  usual  forms  of  support  is  a  move  in  the 
wrong  direction.  A  structure  40  ft.  or  more  in  height  with  only 
the  resistance  to  bending  inherent  in  such  members  acting  as 


FIG.  72. — A-frame  trolley  poles.     European  installation. 

cantilever  beams  has  naturally  very  much  more  flexibility  than 
is  required.  To  balance  wire  tensions  only  a  slight  movement 
of  the  pole  top  is  required.  Narrow-base  A-frames  have  some- 
times erroneously  been  termed  poles  or  semi-flexible  structures. 
A  pole  or  tower  is  an  enclosing  or  box-girder  structure  with  four 
planes  of  bracing,  whereas  the  narrow-base  A-frame  or  latticed 
channel  has  but  one  central  plane  of  bracing  and  is  a  true  flexible 
frame. 

Assuming  that  a  reasonable  amount  of  skill  has  been  employed 
in  selecting  spans,  heights  and  main  sections,  the  next  most 
important  step  in  building  an  adequate  A-frame  line  is  to  pro- 


130 


POLE  AND  TOWER  LINES 


vide  an  overhead  ground  wire  and  substantial  foundations. 
The  ground  wire,  which  should  have  considerable  strength, 
may  be  given  a  little  less  sag  than  the  conductors  so  it  will 
serve  as  a  continuous  head  guy,  the  value  of  which  can  hardly  be 
overestimated.  In  fact,  it  is  difficult  to  string  the  conductors 
unless  there  is  a  ground  wire  in  place  to  steady  the  frames. 

The  foundations  are  also  of  great  impor- 
tance, since  flexible  frames  are  not  well  adapted 
to  withstand  eccentric  loading.  If  the  base  of 
one  main  leg  settles,  or  is  erected  at  a  different 
level  than  the  other,  the  deviation  of  the  top 
of  the  frame  will  be  about  seven  times  as  much 
as  the  settlement,  depending  on  the  height  and 
spread  at  the  base.  Moreover,  as  the  failure 
of  a  frame  will  usually  result  from  the  buckling  of  the  main  chan- 
nels, anything  which  disturbs  an  equal  distribution  of  stress  be- 
tween the  legs  will  promote  failure. 

That  considerable  foundation  stress  may  be  developed  is 
shown  by  the  fact  that  in  a  number  of -tests  the  bent  rods  used 
to  attach  the  anchor  members  have  been  straightened  out  at 
the  bend  (Fig.  73).  It  seems  probable,  therefore,  that  more 
rigid  attachments  are  needed. 


FIG.  73. 


FIG.  74. 

The  conditions  which  produce  buckling  are  not  very  clearly 
understood,  or  rather  their  limits  are  not  definitely  known.  If 
the  main  channels  are  assumed  to  be  made  of  absolutely  identical 
material  and  the  base  of  the  foundation  is  firm  and  unyielding, 
some  difference  in  the  lateral  supports  at  the  ground  line  or  in 
the  rigidity  of  the  bracing  connections  may  allow  sufficient 


STEEL  POLES  AND  TOWERS 


131 


deflection  to  start  buckling.  As  the  failure  is  a  compressive 
failure  in  a  relatively  long  column,  any  measures  which  restrain 
the  main  legs  from  moving  sideways  at  any  point  will  be  of 
effective  service.  A  comparatively  long  stiff  connection  of  the 
bracing  with  the  main  legs  is  useful  as  it  stiffens  the  column 
locally.  Such  connections,  therefore,  should  never  be  made  with 
less  than  two  rivets,  and  should  preferably  be  not  less  than  6 
in.  long.  Further,  the  diagonal  braces  should  not  have  any 
slack,  and  if  made  of  rods  or  other  adjustable  members,  should  be 
tightened  as  near  equally  as  possible. 

One  of  the  most  important  requirements  in  A-frame  construc- 
tion is  to  provide  guys  at  all  corners  above  about  3°,  and  to  pro- 


FIG.  75. — A-frame  failure. 

vide  strain  towers  at  corners  above  10°.  This  is  in  addition  to  a 
more  or  less  definite  number  of  head  guys  on  tangents.  In  wire 
stringing,  it  is  almost  impossible  to  pull  out  the  wires  unless 
an  overhead  ground  wire  has  been  previously  clamped  in  posi- 
tion to  steady  the  frames.  It  is  further  necessary  to  pull  all 
three  wires  of  a  circuit  at  one  time,  using  a  dynamometer  and 
an  equalizing  rig  to  balance  the  tension  in  the  wires.  If  an 
attempt  is  made  to  string  one  wire  at  a  time,  the  frames  may  be 
twisted  by  the  unbalanced  loading.  It  is  entirely  possible  to 
design  and  construct  A-frame  lines  which  will  be  satisfactory  in 
cost  and  operation,  but  this  cannot  be  done  without  the  exercise 
of  care  and  skill  both  in  the  design  and  in  the  erection. 


132 


POLE  AND  TOWER  LINES 


The  present  tendency,  and  the  writer  ventures  to  believe  it  a 
proper  tendency,  is  toward  the  use  of  galvanized  ground-stub 
angles,  whether  the  superstructure  is  painted  or  galvanized  and 
with  either  concrete  or  earth  back  filling.  Galvanizing  such 
members  is  a  relatively  inexpensive  operation.  If  desired, 
the  galvanized  surface  can  be  painted  over  at  the  ground  line. 
No  reduction  of  section  on  account  of  the  protective  coating 
should  be  permitted  in  the  ground  stubs. 

As  flexible  frames  are  painted  as  readily  as  narrow-base  poles, 
the  cost  being  in  the  neighborhood  of  $2  per  structure,  there 
is  no  objection  as  far  as  cost  is  concerned  to  the  use  of  painted 
rather  than  galvanized  superstructures. 

RECORD  OP  FLEXIBLE  A-FRAMES.     REPORTS  (1915)  FROM  Six  LINES  HAVING 
A  TOTAL  OP  958  FRAMES  IN  SERVICE 


Number           A.frame 
A-frames           failures 

Remarks 

158             None 
262             None 
62             None 
100             None 
290                 3 
86               56 

Two  cross  arms  twisted. 
Four  broken  insulator  connections. 
Foundations  pulled  up. 
Storm  of  Dec.,  1914. 

958 


STEEL  POLES  AND  TOWERS 


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CHAPTER  VII 
SPECIAL  STRUCTURES 

Transposition. — In  addition  to  the  regular  line  structures, 
provision  must  be  made  for  supports  on  which  the  conductors 
may  be  transposed.  Transposition  poles  or  towers  are  few  in 
number  and  their  divergence  from  the  standard  type  may  or 
may  not  be  marked.  In  some  cases  it  is  only  necessary  to  change 
the  attachment  of  the  conductors  to  a  different  one  of  the  usual 


FIG.  76. — Wooden  pole  terminal  rack. 

points  of  connection.  In  other  cases  structures  with  greater 
conductor  spacing  or  specially  arranged  crossarms,  etc.,  may  be 
required.  Under  any  circumstances,  whether  the  normal 
separation  be  small  or  great,  care  must  be  taken  not  to  unduly 
diminish  the  clearance  between  conductors  where  they  cross  in 
the  span. 

141 


142 


POLE  AND  TOWER  LINES 


Structurally,  transposition  consists  of  interchanging  the  points 
of  conductor  attachment,  so  that  no  one  conductor  maintains 
its  original  location  throughout  the  entire  line.  The  number  of 
transpositions  and  the  distances  between  the  points  at  which  they 
are  made  vary  greatly  for  different  installations,  voltages,  etc. 
It  is  possible  that  the  introduction  of  special  transposition  struc- 


FIG.  77. — Outdoor  sub-station. 

tures  might  be  reduced  by  the  combination  of  the  transposition 
towers  with  special  towers  otherwise  required  at  various  points. 
A  more  frequent  form  of  transposition  is  that  customary  in  the 
telephone  wires  carried  on  transmission  line  supports.  This  may 
be  done  at  every  few  poles  on  short-span  lines  or  in  every  span  on 
long-span  lines.  Owing  to  the  smaller  clearances  required  be- 
tween such  wires,  however,  the  measures  for  their  transposition 


SPECIAL  STRUCTURES 


143 


will  frequently  consist  merely  of  providing  two  points  about  a 
foot  apart  vertically  for  the  attachment  of  the  arm  or  bracket 
angle. 

Another  method  is  to  cross  the  wires  inside  the  tower,  by 
using  pins  of  different  height  or  placing  one  pin  on  a  raised 
shelf  angle. 

Outdoor  Sub -stations. — Outdoor  sub-stations  in  which  the 
apparatus  is  mounted  on  poles,  towers,  or  frames,  instead  of 
being  inclosed  in  a  building,  have 
come  into  considerable  use  in  the 
last  few  years,  although  the  pro- 
priety of  their  use  under  adverse  cli- 
matic conditions  has  been  the  sub- 
ject of  criticism.  Such  stations  are 
of  various  types,  depending  on  the 
voltage,  location,  and  importance  of 
the  line.  In  the  majority  of  in- 
stances, the  transformers  have  been 
elevated  and  supported  on  plat- 
forms from  10  to  15  ft.  above  the 
ground.  By  so  doing,  the  trans- 
formers are  removed  from  the  zone 
of  possible  contact  with  passersby, 
but  at  the  expense  of  accessibility 
and  with  the  addition  of  heavier  and 
more  costly  supports.  When  the 
transformers  are  placed  at  the 
ground  level,  it  is  frequently  desir- 
able to  inclose  them  in  metal  or  wire 
screens  in  order  to  isolate  them  from 
contact. 

Switching  Stations,  Etc. — Outdoor 
switching  stations,  transformer  sta- 
tions,   etc.,  ^have    come    into   very  FlG   78._outdoor  sub-station, 
general  use  in  some  form  or  another 

in  recent  years.  In  the  simplest  type  they  consist  of  special 
arms,  platforms,  etc.,  on  one  or  more  wood  poles,  involving  no 
particular  changes  from  the  standard  line  construction.  The 
more  elaborate  designs  are  dead-ending  frames,  of  timber  or  steel, 
with  special  supports  for  insulators,  switches,  arresters,  etc.,  and 
platforms  or  housing  for  transformers. 


144 


POLE  AND  TOWER  LINES 


The  very  large  clearances  necessary  with  high  voltages  have 
been  the  cause  of  a  number  of  rather  pretentious  structural 
frames.  In  form,  these  towers  or  series  of  connected  towers,  as 
the  case  may  be,  have  not  adhered  to  any  particular  outline,  their 
shape  usually  depending  on  local  conditions  and  requirements. 
Inasmuch  as  such  structures  usually  serve  as  dead-end  supports, 
or  worse,  as  sharp-corner  towers,  and  have  a  large  area  exposed 
to  the  wind,  they  should  be  relatively  much  stronger  than  the 


FIG.  79. — Single-circuit  tower,  flat  spacing  (emergency  switches). 

standard  line  supports.  There  has  unquestionably  been  some 
tendency  toward  the  use  of  excessive  length  ratios,  and  of  incon- 
sistent connections  at  the  foundation.  It  must  be  remembered 
that  a  structure  which  terminates  at  the  ground  line  in  a  single 
angle  at  each  corner  requires  those  angles  to  transmit  all  the  shear 
in  addition  to  their  tensile  or  compressive  stresses.  Such  con- 
struction frequently  appears,  and  in  fact  is,  top  heavy. 


SPECIAL  STRUCTURES 


145 


High  Towers. — Unusually  high  towers  are  required  at  river  cross- 
ings, or  where  one  or  both  ends  of  a  very  long  span  are  on  low 


FIG.  80. — Switching  frame. 


FIG.  81. 

ground.  In  the  former  case  permission  to  make  the  crossing, 
if  over  a  navigable  stream,  must  be  obtained  from  the  War  De- 
partment. The  requirements,  heretofore,  have  been  merely  that 


10 


146 


POLE  AND  TOWER  LINES 


there  should  be  no  encroachment  on  government  lines,  and  that  a 
certain  minimum  overhead  clearance  be  provided.  The  clear- 
ance varies  with  the  importance  of  the  stream,  ranging  from  120 
ft.  over  creeks  or  small  rivers  to  150  ft.  over  deep-water  high- 
ways. The  structures  for  such  crossings  may  be  of  three  types: 
guyed  masts,  semi-dead-end  towers,  and  dead-end  towers. 

The  desirability  of  any  one  of  these,  for  a  particular  crossing, 
will  depend  on  the  number  of  wires,  the  ground  space  available, 


FIG.  82. — Guyed  masts  151  ft.  high,  28  wires. 

and  the  importance  of  the  line.  If  the  character  of  the  adjoining 
supports  is  such  that  they  may  be  depended  on  to  maintain  a 
stretch  of  wires  of  which  a  reasonable  percentage  is  unbroken, 
then  there  is  no  need  of  the  high  structures  being  "self-support, 
ing."  If  only  a  limited  ground  space  can  be  obtained  recourse 
must  be  had  to  guyed  masts,  the  strength  of  which  lies  in  the  un- 
broken wires  and  the  guys.  Towers  capable  of  completely  dead- 
ending  a  small  number  of  wires  are  entirely  feasible,  but  to  dead- 
end heavy  lines  with  no  allowance  for  pullback  or  guys  would 
require  considerable  expense  and  massive  structures. 


SPECIAL  STRUCTURES 


147 


The  towers  in  Figs.  82,  83,  and  84  support  probably  the  heaviest 
lines  thus  far  carried  in  overhead  river  crossings.  None  of  these 
structures  was  designed  to  dead-end  the  full  number  of  wires 
under  J£  in.  ice  and  8.0  Ib.  wind  load.  They  were,  however,  de- 
signed to  withstand  the  maximum  transverse  loading,  combined 
with  some  unbalancing,  such  as  a  few  broken  wires,  with  a  factor 
of  safety.  Since  the  normal  wire  tension  is  approximately  only 
one-third  of  the  maximum,  they  would  also  presumably  withstand 
normal  dead-ending.  Some  additional  security  is  obtained  by 
longitudinal  guys  away  from  the  river,  since  failure  of  many  wires 
could  result  only  from  accident  to  adjoining  structures.  It  may 


FIG.  83. — Semi-dead-end  towers. 

be  noted  that  there  is  no  grading  up  of  the  adjoining  supports, 
the  rise  of  the  wires  to  the  high  towers  being  made  in  one  span. 
This  method  was  first  adopted  as  a  standard  construction  on 
these  installations,  and  the  results  have  been  entirely  satisfactory. 
Since  the  change  in  elevation  of  the  wires  is  from  a  height  of 
about  50  ft.  to  one  of  150  ft.,  it  is  evident  that  a  number  of  high 
approach  towers  are  avoided.  Such  an  abrupt  inclination  of  the 
wires  is  unsuited  to  pin  insulators  because  the  wires  would  not 
clear  the  petticoats  in  descending  and  would  pull  the  insulators 
from  the  pins  in  ascending,  except  that  a  saddle  covering  a  double 


148 


POLE  AND  TOWER  LINES 


series  of  pin  insulators  might  be  used.  The  most  satisfactory 
arrangement,  and  the  strongest  mechanically,  is  to  use  strain 
insulators  of  the  types  shown  in  Figs.  115  and  117. 

On  the  high  towers  the  insulators  may  be  in  the  strain  position 
or  they  may  be  suspended.  On  the  adjoining  supports  they  may 
be  in  the  strain  position  or  in  the  suspended  position  under  the 
wire.  In  the  latter  case  the  pole  or  tower  next  to  the  river  tower 
is  generally  subjected  to  uplift  and  has  no  downward  load. 

Care  must  be  exercised  in  the  design  and  in  the  wire  stringing 
to  balance  the  tension  in  the  crossing  and  adjoining  spans,  other- 


Fio.  84. — Semi-dead-end  towers. 

wise  the  high  towers  will  be  subjected  to  a  heavy  loading  and  the 
underhung  insulators  noticeably  deflected  from  the  vertical. 
When  the  crossing  span  is  much  longer  than  the  approach  spans, 
as  is  frequently  the  case,  the  wires  in  the  inclined  span  will  have 
very  small  sags  and  may  be  brought  together  on  the  low  pole  to 
the  standard  spacing  for  the  voltage  in  question,  even  though  a 
considerable  separation  is  used  to  prevent  swinging  contacts  in 
the  long  span. 

All  high  towers  should  be  provided  with  overhead  ground  wires 
and  ladders,  and   should   be  grounded  below  the  foundations. 


SPECIAL  STRUCTURES 


149 


As  these  structures  are  relatively  heavy  it  is  more  economical 
to  paint  them  than  to  use  galvanized  material,  the  saving  in 
original  cost  being  more  than  ample  to  maintain  a  high  degree 
of  protection  with  paint. 

Aerial  Cable. — In  some  instances  when  an  overhead  line  has 
had  to  cross  territory  subject  to  restrictive  requirements,  and 
particularly  when  adjacent  to  submarine  or  duct  lines,  aerial 
insulated  cables  have  been  used.  This  arrangement  is  practicable 


FIG.  85. 


Aerial  cable. 


FIG.  86. 


only  for  voltages  below  about  22,000,  because  satisfactory  in- 
sulated cable  is  not  obtainable  for  higher  voltages. 

In  the  installation  shown  in  Figs.  85  and  86,  the  cables  are 
hung  from  steel  messengers  and  supported  on  a  low  line  of 
closely  spaced  poles.  The  clearances  necessary  are  those  due 
to  the  physical  requirements  for  passageway  beneath  the  cables. 
Short  heavy  poles  are  used,  being  guyed  at  corners  and  ends 
but  with  no  attempt  to  design  for  broken  messengers  or  cables. 


150 


POLE  AND  TOWER  LINES 


In  the  background  of  Fig.  86  may  be  seen  a  heavy  H-frame, 
telegraph  and  telephone  line.1 

An  aerial  duct  line  for  13 ,000- volt,  three- wire,  insulated  cables, 
attached  to  the  side  of  a  railroad  bridge,  is  shown  in  Fig.  87. 
Provision  was  made  for  three  circuits  and  one  spare.  As  shown, 


FIG.  87. — Aerial  duct  line. 


the  cables  are  encased  in  split  metal  tubes  supported  on  steel 
brackets,  riveted  to  the  outer  stiffeners  of  the  bridge.  This 
construction  was  considered  advisable  as  the  line  had  to  cross 
two  railroads,  one  heavy  high-voltage  line  on  tall  towers,  and 
several  very  important  telephone  and  telegraph  systems,  and 

Subsequently  wrecked  by  a  very  severe  sleet-storm. 


SPECIAL  STRUCTURES  151 

there  was  not  enough  space  available  between  the  existing  power 
and  telegraph  systems. 

In  making  such  attachments  to  bridges,  and  to  some  extent  to 
any  foreign  structures,  the  construction  must  be  accessible  with- 
out interference  with  other  interests,  and  at  the  same  time  be 
free  from  the  probability  of  injury  by  foreign  workmen. 


CHAPTER  VIII 
CONCRETE  POLES 

Wood  poles  had  and  still  have  certain  advantages;  they  also 
have  certain  disadvantages.  Some  of  the  good  points  cannot 
be  duplicated  in  concrete,  but  on  the  other  hand,  some  of  the 
objectionable  features  can  be  eliminated.  Therefore,  omitting 
undue  enthusiasm  on  the  one  hand  and  any  pretensions  to  magic 
excellence  on  the  other,  the  matter  is  purely  and  simply  one  of 
final  cost  and  final  efficiency. 

It  is  self  evident  that  concrete  poles  can  be  made,  and  it  is 
fairly  well  known  that  a  few  thousand  have  been  made  and  are 
now  in  service.  The  only  remaining  considerations  seem  to 
be  ones  of  mechanical  efficiency  and  actual  or  proper  cost. 
The  question  of  mechanical  efficiency  is  in  reality  combined 
with  that  of  cost.  Given  time  and  money  enough  there  are 
few  structures  which  cannot  be  constructed,  even  by  an  amateur. 
Economical  construction  is  another  matter,  and  by  economical 
is  meant  true  economy — final  economy — not  merely  reduced  first 
cost.  Thus,  in  the  case  of  pole  construction,  we  find  that  some 
have  been  built  at  a  low  initial  cost  but  with  an  equally  low 
mechanical  efficiency,  while  others  have  been  built  at  abnormal 
expense  and  with  excessive  strength.  Neither  extreme  is  good 
engineering  nor  good  economics. 

If  an  important  wood  line  is  to  be  built,  stronger  poles  are  used 
than  for  an  unimportant  line.  If  the  species  of  timber  has 
great  strength  and  the  poles  are  " selected,"  smaller  poles  may 
be  used  than  if  the  timber  is  sappy,  knotted,  etc.  Likewise,  if 
concrete  poles  are  to  be  used,  the  importance  of  the  work  and 
the  dependability  of  the  concrete  should  influence  the  specified 
size  and  strength.  Further,  and  in  view  of  the  very  rare  me- 
chanical failures  of  undecayed  timber  poles,  it  seems  essential 
to  consider  the  actual  strength  of  the  poles  which  are  and  have 
been  carrying  various  installations  of  wires.  The  strength  may 
have  been  too  great — it  was  presumably  not  too  small — but  it 
serves  in  some  degree  as  a  measure  of  the  desirable  strength 
for  concrete  poles  which  may  properly  replace  timber  ones. 

152 


CONCRETE  POLES 


153 


This  method  neglects  the  relative  factors  of  safety,  reference 
to  which  will  occur  elsewhere,  but  it  is  a  fact  that  those  who  will 
without  hesitation  use  any  mean,  crooked  little  timber  pole  will 
develop  a  multitude  of  rules  and  requirements  when  building  a 
concrete  pole. 


FIG.  88. — Hollow  concrete  poles.    European  transmission  line. 

The  writer  does  not  wish,  in  any  degree,  to  convey  approval  of 
weak  or  shoddy  construction,  but  would  emphasize  the  need  for 
intelligent  analysis  of  the  subject.  Whatever  the  future  of 
concrete  poles,  it  can  be  of  no  service  to  the  power  or  telegraph 
business  to  construct  either  weak  poles  or  ridiculously  strong 
ones.  It  cannot  be  denied  that  individual  success  in  the  con- 


154  POLE  AND  TOWER  LINES 

crete-pole  industry  will  depend  very  largely  on  the  degree  in 
which  the  fabricator  is  successful  in  maintaining  a  uniform 
product. 

In  the  case  of  wood  poles,  there  has  always  been  a  wide  range 
in  the  strength  even  within  the  same  species,  greater  differences 
than  are  perhaps  generally  realized.  These  differences  together 
with  decay  have  been  guarded  against  by  large  safety  factors — or 
perhaps  more  accurately  by  the  use  of  apparently  large  factors — 
the  factors  being  actually  less  than  supposed.  In  many  cases 
there  has  been  the  utterly  indefensible  practice  of  assuming 
impracticable  loadings  together  with  impossible  unit  stresses. 
There  can  be  no  engineering  justification  for  claiming  that 
poles  are  designed  to  withstand  certain  improbable  loads  with 
a  factor  of  safety  of  5  or  6  when,  in  fact,  99  per  cent,  of  the 
poles  would  not  be  subjected  to  such  loads,  nor  could  they  carry 
anything  like  five  times  such  loads. 

When  steel  is  embedded  in  well-made  concrete  its  preservation 
is  perfect  and  the  life  of  a  reinforced  monolith  is  practically  in- 
definite. If  designed  and  built  with  the  same  attention  now 
given  other  materials,  reinforced-concrete  poles  should  attain 
the  necessary  strength  and  give  satisfactory  service.  Like  steel 
poles,  they  can  be  spaced  greater  distances  apart  than  is  eco- 
nomically possible  with  wood  poles,  and  their  fire-resisting  quali- 
ties are  about  equal  to  steel  poles.  This  latter  feature  will 
become  of  increased  importance  with  the  spread  of  modern 
requirements  for  fire  protection. 

In  damp  climates,  or  in  localities  where  wood  poles  are  subject 
to  attack  by  fungi,  or  insects,  concrete  poles  have  a  longer  life 
than  either  steel  or  timber.  By  the  insertion  of  pipes,  or  the 
formation  of  an  axial  passage  in  the  concrete,  wires  may  be 
carried  from  the  pole  tops  to  the  ground  and  thence  in  any 
desired  direction  and  are  thus  entirely  protected  at  little  addi- 
tional cost. 

Owing  to  the  natural  taper  of  timber,  it  is  frequently  the 
case  that  the  weakest  section  is  at  some  point  above  the  ground 
level.  Therefore,  there  is  an  excess  of  material  in  the  butt, 
which  may  be  considered  wasted,  except  in  so  far  as  this  surplus 
timber  is  useful  in  resisting  decay.  A  reinforced-concrete  pole 
may  be  given  any  desired  taper  and  need  have  little  or  no  excess 
of  improperly  placed  material. 

If  we  may  judge  by  the  kind  of  handling  which  concrete  piles 


CONCRETE  POLES 


155 


successfully  withstand,  it  seems  entirely  probable  that  concrete 
poles,  if  properly  reinforced,  will  survive  any  shocks  incident 
to  ordinary  service.  When  subjected  to  an  overload  or  acci- 
dental shock,  a  timber  pole  will  bend  and  in  some  cases  survive; 
but  failure  when  it  does  occur  is  usually  complete  and  the 
pole  falls.  Concrete  poles,  on  the  contrary,  while  without  the 
elasticity  of  timber,  do  not  fail  by  breaking  off,  but  are  held  by 
the  reinforcement  from  falling  to  the  ground.  Tests  also 


FIG.  89.  FIG.  90. 

Concrete  poles. 

show  that  a  reasonable  amount  of  bending  (sufficient  to  balance 
stresses  in  the  wires)  can  occur  without  apparent  injury  to  the 
pole. 

The  chief  cause  of  skepticism  heretofore  has  been  the  fear  that 
such  long,  slender  concrete  beams  would  not  be  able  to  withstand, 
without  cracking,  the  bending  stresses  and  measurable  deflections 
of  a  pole  line.  If  the  poles  are  properly  designed,  cracks  due  to 
partial  failure  will  not  occur.  Hair  cracks  are  of  infrequent 
occurrence,  microscopic  in  character,  and  experience  has  ap- 
parently shown  that  they  will  not  admit  moisture  in  sufficient 
quantity  to  injure  a  reinforced-concrete  structure. 


156  POLE  AND  TOWER  LINES 

Considerable  misinformation  has  been  published  on  the  subject 
of  concrete  poles.  Generally  this  has  taken  the  form  of  cost 
data,  which  could  not  be  duplicated  in  actual  practice.  The 
plain  fact  of  the  matter  is  that  something  cannot  be  obtained 
for  nothing  even  in  concrete.  If  a  concrete  pole  of  considerable 
strength  is  required,  the  material  needed  to  provide  that  strength 
must  be  purchased  and  placed  in  the  pole.  No  costs  are  directly 
comparable  unless  they  relate  to  poles  of  similar  strength  and 
similar  quantities.  For  instance,  30-ft.  poles  having  an  ulti- 
mate strength  of  about  1000  Ib.  have  been  made  for  approxi- 
mately $8,  but  a  30-ft.  pole  having  a  strength  of  6000  Ib.  would 
require  reinforcement  the  cost  of  which  alone  would  exceed 
the  entire  cost  of  the  weaker  poles. 

The  number  of  poles  required  has  a  very  direct  bearing  on  the 
cost  and  on  the  proper  place  of  manufacture.  The  cost  of  forms, 
plant,  superintendence  and  engineering  must  be  borne  propor- 
tionally by  the  number  of  poles  to  be  made  by  that  manufacturing 
plant.  Again,  the  plant  and  forms  for  a  small  number  of 
poles  would  not  be  ultimately  economical  for  a  large  installa- 
tion. Too  little  attention  has  been  paid  to  the  very  re&l  cost  in 
time  and  money  required  to  properly  develop  the  engineering 
side  of  the  question.  It  is  not  beyond  the  bounds  of  reason  to 
believe  that  the  costs  of  investigation  and  engineering  if  included 
in  some  published  data  would  have  more  than  doubled  the  stated 
costs. 

One  criticism  which  can  properly  be  made  of  concrete  poles  is 
that  they  have  considerable  weight.  This  affects  both  the  cost 
of  handling  and  the  amount  of  the  freight,  if  the  poles  are  made 
in  a  distant  yard.  In  an  attempt  to  reduce  this  cost,  poles  have 
been  cast  in  place  and  the  claim  made  that  an  economy  is  ef- 
fected thereby.  Such  procedure  is  open  to  serious  criticism, 
both  on  the  score  of  the  probable  excellence  of  the  final  work 
and  as  to  the  actuality  of  the  economy  claimed.  It  would  seem 
difficult  to  make  any  experienced  constructor  believe  in  the 
economy  of  moving  the  forms,  water  and  aggregates  from  pole 
to  pole  and  casting  in  the  upright  position  with  the  delays  con- 
sequent to  such  an  arrangement. 

A  number  of  what  may  be  termed  freak  designs  have  been  given 
more  or  less  extended  trial,  but  they  appear  to  have  little  to 
commend  them.  Among  these  may  be  mentioned  the  com- 
bination wood  and  concrete  pole  consisting  of  a  wooden  pole 


CONCRETE  POLES 


157 


incased  in  a  shell  of  reinforced  concrete,  the  triangular  or  un- 
climbable  pole,  the  sectional  pole  and  the  type  with  hollow- 
paneled  sides. 

Concrete-incased  timber  has  no  claim  to  efficiency,  either 
theoretically  or  practically  and  it  is  not  believed  that  the 
type  will  ever  be  given  another  trial.  Triangular  concrete  poles 
possess  the  disadvantage  common  to  all  triangular  poles — that 


91. — Coombs  concrete  pole 
(test  pull  6000  Ibs.) 


FIG.  92. — Coombs  concrete  pole 
(test  pull  7200  Ibs.) 


of  having  much  less  strength  in  one  direction  than  in  the  opposite 
direction.  By  the  addition  of  the  small  amount  of  material 
needed  to  form  a  square,  either  a  stronger  pole  is  obtained,  or  a 
reduction  in  outline  or  reinforcement  made  possible. 

Hollow-sided  poles  have  been  used  to  some  extent  in  Germany, 
but  the  only  advantage  which  would  appear  to  justify  their  use 
here  is  reduced  weight,  and  this  can  be  secured  by  using  a  hollow- 


158  POLE  AND  TOWER  LINES 

axis  pole.  Hollow  panels  or  steps  reduce  the  strength  in  one 
direction  and  apparently  impose  a  heavy  arch  duty  on  the  steps 
to  care  for  shear  and  buckling  stresses.  Furthermore,  it  is 
doubtful  whether  such  poles  situated  in  a  locality  subject  to 
snow  and  sleet  would  be  climbable. 

It  has  been  claimed  by  some  that  if  cracks  develop  in  concrete 
poles  they  may  be  readily  filled  with  cement,  the  inference  being 
that  the  pole  is  then  restored  to  the  condition  in  which  it  should 
have  been  originally.  There  is  some  basis  of  fact  in  this  con- 
tention, if  the  cracks  in  question  are  seasoning  cracks.  If,  how- 
ever, the  pole  develops  tension  cracks  due  to  faulty  design,  plas- 
tering with  cement  grout  will  not  prevent  a  repetition  of  the 
cracking  on  a  subsequent  application  of  the  load.  In  addition,  the 
practical  application  of  a  plastering  process  to  poles  in  service  con- 
templates a  rigidity  of  inspection  and  cost  of  maintenance  entirely 
at  variance  with  the  economic  theory  of  the  use  of  concrete  poles. 
In  fact  this  question  of  cracks  is  one  source  of  reasonable  criti- 
cism of  concrete  poles,  as  the  possible  corrosion  of  the  reinforcing 
metal  would  be  a  very  serious  matter.  Thus  it  has  been  pointed 
out  that  owing  to  the  thin  shell  of  concrete  outside  the  rods — 
usually  about  1  in.  thick — there  may  be  absorption  of  moisture 
even  when  no  hair  cracks  or  deflection  cracks  are  present. 

In  some  locations,  notably  that  of  the  Pennsylvania  Railroad 
poles  designed  by  the  writer  for  the  line  on  the  New  Jersey 
meadows,  the  constantly  wet  foundation  and  the  size  of  the  line 
—this  being  the  heaviest  concrete  installation  in  the  country — 
renders  this  possibility  of  importance  in  the  design.  The  soil 
in  question  is  a  peaty  bog  and  always  holds  water.  Above  ground 
there  is  usually  water  during  portions  of  the  year  although  it  is 
fairly  dry  in  summer.  In  designing  the  above  line,  it  was  felt 
that  by  making  the  poles  of  considerable  strength,  providing  for 
local  stresses  and  manufacturing  under  competent  supervision, 
the  probability  of  cracks  or  corrosion  would  be  very  nearly 
eliminated. 

Since  in  solid  poles  of  light  capacity  the  loading  produces  a 
low  compressive  unit  stress  in  the  concrete,  a  considerable  area 
of  concrete  might  be  omitted,  or,  theoretically,  the  economical 
section  would  be  a  hollow  one.  The  relatively  greater  weight  of 
a  solid  pole  makes  it  more  difficult  to  handle;  therefore,  a  hollow 
pole  would  be  more  economical  to  erect.  Further,  the  sides  of 
the  pole  which  resist  bending  stresses  normal  to  the  line  might 


CONCRETE  POLES 


159 


be  at  a  greater  distance  from  the  center  than  the  sides  perpen- 
dicular to  the  line. 

There  are,  however,  certain  objections  to  the  use  of  hollow  or 
unsymmetrical  sections.  The  former  are  difficult  to  make 
properly  and  the  cost  of  the  forms  exceeds  that  required  for  solid 
sections.  Unsymmetrical  sections  may  perhaps  be  open  to 
criticism  on  the  score  of  appear- 
ance and  if  the  lack  of  symmetry 
is  very  pronounced,  the  poles  will 
be  relatively  weak  in  one  direction. 

In  general,  a  square,  octagonal, 
circular  or  other  cross-section  may 
be  used,  but  as  a  matter  of  appear- 
ance it  is  desirable  that  all  corners 
be  chamfered  or  rounded  since 
sharp  edges  are  difficult  to  make 
and  easily  broken.  The  minimum 
diameter,  or  width,  at  the  top  may 
be  made  6  in.  for  small  poles  and 
increased  as  required  for  the 
strength  and  appearance  of  long 
poles  or  poles  carrying  a  heavy 
line.  In  any  case  care  should  be 
exercised,  in  determining  the  taper 
and  reinforcement,  that  no  weak 
section  occurs  at  some  distance 
above  the  ground-level. 

In  regard  to  the  factors  of 
safety,  unit  stresses  and  working 
stresses,  to  be  allowed  in  the  con- 
stituent materials  of  a  reinforced- 
concrete  pole,  there  is  as  much 
latitude  of  judgment  as  in  other 
structural  work.  The  character  of 


FIG.  93.  FIG.  94. 

FIG.  93. — Torsion  test  on  pole 
not  provided  with  spiral  rein- 
forcement— diagonal  crack  near 
top. 

FIG.  94. — European  +  shaped 
pole. 


service  is  not  closely  akin  to  that  of  bridges  or  buildings,  so  the 
factors  of  safety  common  to  such  work  would  be  too  conserva- 
tive for  poles  computed  for  extreme  conditions  of  loading. 

The  present  practice  differs  rather  widely  as  to  the  most 
economical  or  most  desirable  distribution  of  reinforcement,  but, 
it  is  now  generally  conceded  in  reinforced-concrete  work  that  the 
finer  the  distribution  of  metal,  the  greater  will  be  the  homogeneity 


160 


POLE  AND  TOWER  LINES 


and  strength  of  the  construction.  However,  in  the  case  of  poles, 
where  the  concrete  is  deposited  within  narrow  forms,  other  con- 
ditions partly  modify  or  control  the  distribution.  If  the  metal 
is  concentrated  in  four  equal  areas,  a  rod  at  each  corner,  a  square 
pole  will  be  equally  strong  either  parallel  or  normal  to  the  line. 
Other  or  finer  distribution  of  metal  with  equal 
strength  in  both  directions  necessitates  an  ex- 
cess of  material  over  that  required  for  the  forces 
normal  to  the  line.  When  the  metal  is  concen- 
trated, the  fabrication  of  the  reinforcement  into 
a  unit  frame  and  also  the  concreting  operations, 
are  more  easily  accomplished.  It  may  be  said, 
as  in  the  case  of  beams,  that  ample  web  rein- 
forcement assures  a  firm  unyielding  unit  during 
concreting,  as  well  as  provision  against  vertical 
shearing  stresses. 

In  other  fields  of  reinforced  concrete  work 
high-carbon  steel  with  a  high  elastic  limit  and 
a  correspondingly  richer  concrete  are  being  used 
to  permit  high  working  stresses  in  design.  If,  in 
such  work,  high-unit  stresses  can  be  used,  with 
a  percentage  for  impact,  it  seems  entirely  reason- 
able to  allow  correspondingly  high  working 
stresses  in  pole  design,  since  the  severe  condi- 
tions of  loading  occur  infrequently. 

In  the  construction  of  concrete  poles  or  other 
structures  in  which  there  is  a  relatively  large 
and  important  amount  of  reinforcing,  great  care 
must  be  exercised  to  thoroughly  tamp  or  puddle 
the  concrete  as  it  is  deposited,  in  order  to  pre- 
vent pockets  and  to  insure  every  lineal  inch  of 
metal  having  a  firm  adherence  to  the  concrete. 
In  such  structures,  the  increase  in  stress  in  the 
reinforcement  must  be  very  rapid  and  such  ad- 
ditions of  stress  are  dependent  on  the  efficiency 
of  the  connection  between  the  steel  and  the 
concrete.  Mechanical  bond  or  deformed  bars,  i.e.,  twisted 
squares  or  bars  with  various  projections  on  their  surfaces,  are 
superior  to  smooth  bars  for  work  in  which  high  stresses  must 
be  developed  in  short  lengths.  Rods  may  often  be  bent  into 
hooks  or  clamped  together  with  advantage. 


FIG.  95.— 
Well-designed 
concrete  pole 
tested  to  de- 
struction, show- 
ing multiplicity 
of  cracks. 


CONCRETE  POLES  161 

Reinforcing  metal  may  be  either  medium-grade  steel  having  an 
ultimate  strength  of  60,000  to  70,000  Ib.  per  square  inch,  and  an 
•elastic  limit  of  30,000  to  40,000  Ib.  per  square  inch  and  capable 
of  being  bent  cold  about  its  own  diameter,  or  it  may  be  high- 
carbon  steel  of  80,000  to  100,000  Ib.  per  square  inch  and  an 
elastic  limit  of  40,000  to  60,000  Ib.  per  square  inch  and  capable 


FIG.  96. — Concrete  telephone  and  telegraph  poles. 

of  being  bent  cold  without  fracture  about  a  radius  equal  to  four 
times  the  diameter  of  the  rod.  Since  the  elastic  limits  of  these 
two  grades  of  material  are  quite  different  and  since  the  failure 
of  a  concrete  pole  occurs  when  the  tension  surface  cracks,  there 
will  be  no  similarity  between  two  poles  of  the  same  dimensions 
and  amount  of  reinforcement  in  which  different  grade  rods  are 
11 


162 


POLE  AND  TOWER  LINES 


used.  Owing  to  the  fact  that  in  a  pole  the  stresses  in  the  rein- 
forcement must  change  rapidly  in  amount  with  every  lineal  foot 
of  the  pole,  it  is  most  essential,  at  least  for  high-strength  poles, 
to  use  mechanical  bond  or  twisted  bars.  It  is  also  necessary  to 


FIG.  97. — Loading  concrete  poles. 


FIG.  98. — Hollow  concrete  poles. 

provide  diagonal  or  spiral  reinforcing  when  poles  are  to  be  sub- 
jected to  torsion,  although  close  spacing  of  horizontal  ties  will 
be  of  assistance.  Horizontal  ties  are  needed  primarily  to  re- 
strain the  rods  from  local  buckling  with  its  consequent  spalling 


CONCRETE  POLES 


163 


off  of  concrete.  The  rods  must  be  tied  to  the  horizontal  straps 
or  other  secondary  system  at  each  intersection,  in  order  to  assist 
in  developing  bond  stresses.  In  view  of  the  character  of  service 


FIG.   99. — Concrete  distribution 
line  poles. 


FIG.  100. — Concrete  transmission 
line  poles. 


to  which  horizontal  bands  or  spacers  are  subjected,  it  is  utterly 
indefensible  to  use  cast  rings  or  bands. 

No  attempt  should  be  made  to  remove  the  forms  until  the  con- 


164  POLE  AND  TOWER  LINES 

crete  has  obtained  a  good  set  and  care  must  be  exercised  even 
then  to  prevent  injuring  the  surfaces  during  such  removal.  The 
forms  should  be  kept  covered  during  setting,  particularly  when 
exposed  to  direct  sunlight  in  hot  weather.  After  the  forms  have 
been  removed  the  concrete  pole  should  be  well  sprinkled  and  kept 
under  canvas  for  some  days.  A  freshly  made  pole  cannot  be 
handled  or  rolled  with  impunity  until  it  has  become  well  set. 
Further,  the  subsequent  handling,  particularly  of  long  poles,  must 
be  done  with  care  and  preferably  with  slings  attached  at  two 
separate  points. 

The  bolt  holes  and  step-bolt  sockets  must  be  cast  in  place. 
Hardwood  blocks  may  be  used  for  step  bolts,  but  a  cast  or 
spiral  socket  is  preferable.  Plastering  the  surface  of  poles  to 
remove  pockets  or  to  produce  a  finished  surface  is  particularly 
objectionable.  The  former  should  be  avoided  by  proper  work- 
manship in  the  first  place.  The  latter  is  entirely  unnecessary 
since -a  very  fine  surface  can  be  readily  produced  by  rubbing. 

The  most  commonly  used  mixture  is  1:  2:  4  Portland  cement, 
sand,  and  broken  stone  or  gravel.  It  should  be  mixed  wet, 
using  carefully  selected  materials,  and  tamped  or  churned  to 
eliminate  air  bubbles,  obtain  a  good  surface,  and  thorough 
contact  with  the  reinforcement.  Such  a  mixture  when  well 
made  has  an  average  compressive  strength  of  about  1000  Ib. 
per  square  inch  in  seven  days,  2400  Ib.  per  square  inch  in  one 
month,  3000  Ib.  per  square  inch  in  three  months,  and  3500  Ib.  per 
square  inch  in  six  months.  If  conditions  make  it  desirable  to  use 
high  working  stresses,  a  month  or  more  should  be  allowed  to 
elapse  before  new  poles  are  subjected  to  severe  tests. 


CHAPTER  IX 
FOUNDATIONS 

The  proper  penetration  of  wood  poles  is  the  result  of  many 
years  of  actual  experience  under  the  varying  conditions  of  dif- 
ferent soils.  It  is  not  a  matter  which  can  be  determined  accu- 
rately and  readily  by  a  mathematical  formula.  In  an  ideal 


FIG.  101. 

formula  there  must  be  a  variable  "constant,"  the  value  of 
which  depends  on  the  particular  soil  in  which  the  poles  are  to-be 
erected.  Inasmuch  as  it  is  impracticable  to  make  preliminary 

165 


166 


POLE  AND  TOWER  LINES 


tests  of  the  soil  conditions  on  a  long  transmission  line,  it  is 
necessary  either  to  have  several  standards  or  to  design  a  single 
foundation  which  will  provide  safely  for  ordinary  variations  in 
the  soil. 

In  an  attempt  to  reduce  the  initial  cost  of  installations  a 
number  of  lines  have  been  built  with  weak  foundations.  Some 
of  these  supports  have  already  failed  and  the  length  of  service 

,    of  others  is  a  matter  of  conjecture.     It 

is  true  that  the  cost  of  foundations  for 
wide-base  structures  may  be  compara- 
tively high,  but  the  insurance  value  of 
a  good  foundation  justifies  its  cost. 
The  use  of  shallow  foundations  is  doubt- 
less due  in  a  measure  to  the  methods 
in  vogue  in  testing  towers.  A  test  tower 
on  a  concrete  or  metal  foundation  not 
only  gives  no  information  in  regard  to 
the  subsequent  foundation,  but  induces 
a  false  sense  of  security  both  in  the  prob- 
able action  of  the  foundation  and  of  the 
tower  itself. 

A  foundation  has  two  functions  to 
perform:  first,  to  prevent  uplift  or  de- 
pression, and  second,  to  resist  buckling 
at  the  point  of  maximum  leverage. 
Therefore,  a  foundation  member  or 
ground  stub  which  is  not  firmly  braced 
against  horizontal  movement  at  the 
ground  line  may  introduce  stresses  not 
contemplated  by  the  designer.  A  test 
tower  on  a  rigid  foundation  will  give 


FIG.  102. — Bog  shoe. 


higher  test  values  than  the  line  towers  in  actual  service,  and  the 
test  is,  therefore,  deceptive  by  the  amount  representing  the 
effect  of  this  rigidity.  In  structures  in  which  the  main  legs  are 
unsupported  for  relatively  large  values  of  l/r,  or  in  which  the  tower 
itself  has  much  less  strength  in  one  direction  than  in  the  other, 
a  rigid  anchorage  is  necessary  to  provide  the  conditions  assumed 
in  the  design.  In  some  soils  certain  types  of  poles  and  towers 
have  a  strength  in  excess  of  that  of  the  foundation,  the  result 
being  that  the  weakest  part  of  the  line  is  not  even  suspected  until 
failure  occurs.  There  are  no  pole  penetrations  of  less  than  5  ft. 


FOUNDATIONS 


167 


in  the  standard  pole  settings  for  wooden  poles,  but  some  metal 
structures  have  been  designed  with  penetrations  of  3  ft.  6  in., 
which  is  not  as  deep  as  the  ordinary  frost  line. 

On  the  other  hand,  the  enlarged  butts  of  latticed  steel  poles 
encased  in  concrete  would  warrant  some  reduction  in  their  depth 
of  penetration  as  compared  with  that  of  similar  wood  poles  not 
so  incased. 

The  protection  of  the  metal  in  the  anchorage  is  naturally  of 
vital  importance  to  the  permanence  of  the  structure.  If  there 
are  some  localities  in  which  galvanizing  is  no  real  protection, 
there  are  other  places  where  galvanizing  is  more  economical 


FIG.  103. — Bog  shoe. 

than  paint.  When  the  anchorage  is  incased  in  concrete  "the 
incased  portion  may  safely  be  considered  as  having  a  longer 
life  than  the  superstructure.  The  point  of  entrance  of  the 
metal  in  the  concrete  is  usually  considered  as  the  location  of  the 
future  maximum  corrosion,  whether  the  structure  is  painted  or 
galvanized.  The  writer  believes  that  this  assumption  is  not 
always  correct,  and  that  the  location  of  future  deterioration  will 
depend  on  the  relative  effect  of  acids,  etc.,  carried  by  air  currents 
to  the  upper  portions  of  the  structure,  versus  the  amount  of  dirt 


168 


POLE  AND  TOWER  LINES 


and  water  at  the  base.  However,  there  can  be  no  question  of 
the  propriety  of  protecting  ground  metal,  and  data  are  needed 
on  the  results  obtained  by  galvanizing,  concrete,  asphalt,  tar, 
treated  burlap,  additional  coats  of  paint,  etc.  With  poles  and 
towers  as  in  any  other  kind  of  construction,  a  poor  foundation 
induces  a  poor  superstructure,  and  it  is  probable  that  more 
tower  failures  have  been  caused  or  superinduced  by  faulty  foun- 
dation design  than  by  any  other  cause. 

Wood  poles  should  be  set  in  the  ground  to  depths  not  less  than 
those  specified  in  the  following  table: 

TABLE  29. — WOOD-POLE  SETTINGS 


Length  over  all   (feet) 

Straight  lines   (feet) 

Curves,     corners,      dead 
ends,  etc.  (feet) 

30 

5.0 

6.0 

35 

5.5 

6.0 

40 

6.0 

6.5 

45 

6.5 

7.0 

"     50 

6.5 

7.0 

55 

7.0 

7.5 

60 

7.0 

7.5 

65 

7.5 

8.0 

.  70 

7.5 

8.0 

75 

8.0 

8.5 

80 

8.0 

8.5 

In  rock  excavation  smaller  penetrations  may  be  permitted,  par- 
ticularly if  the  back-fill  is  of  concrete.  If  the  filling  is  of  earth, 
or  the  fine  waste  from  the  excavation,  it  should  be  well  tamped 
into  place.  The  character  of  the  rock  must  be  taken  into  con- 
sideration in  decreasing  the  standard  settings,  since  rotten  rock 
with  a  horizontal  or  inclined  cleavage  may  be  but  little  stronger 
than  the  better  grades  of  packed  earth. 

Although  the  vast  majority  of  pole  foundations  are  included  in 
the  very  elastic  term  "earth,"  it  is  occasionally  necessary  to  set 
poles  in  bad  ground.  There  are  many  varieties  of  uncertain 
soils  and  no  one  design  is  absolutely  adequate  for  all  conditions. 

Light  shifting  sand,  peat  bog,  wet  clay  and  black  muck  are  of 
different  genera  and  frequently  of  different  species.  The  problem 
usually  includes  the  necessity  of  providing  for  a  lack  of  proper 
lateral  resistance  at  the  ground  line,  and  it  may  or  may  not 
include  a  protection  against  settlement. 


FOUNDATIONS 


169 


The  simplest  foundation  reinforcement,  and  one  also  used  to 
strengthen  a  heavily  stressed  pole  in  good  ground  is  the  pro- 
vision of  a  crib  or  toe  and  heel  braces.  A  common  error  in  con- 
structing such  braces  is  that  of  a  weak  connection  to  the  pole. 
Inasmuch  as  the  ground  on  one  side  of  the  pole  is  not  exactly  like 
that  against  which  one  end  of  the  brace  presses,  there  is  always 
some  tendency  to  rotate.  If  a  high  resistance  must  be  developed, 
it  generally  requires  very  considerable  earth  pressures  over  a 


Wooden  Pole. 
Gravel,  Stone 


Wooden  Pole 


Backfill 


FIG.  104. — Barrel  foundation. 


FIG.  105. 


restricted  area.  The  earth  pressures  are  available  if  properly 
developed,  and  they  are  much  greater  than  is  often  supposed, 
the  writer  having  seen  1-in.  bolts  sheared  through  the  wood 
across  the  grain  of  a  3-in.  yellow-pine  plank,  by  the  pressure 
from  a  bank  of  wet  gravelly  clay. 

Anti-tilting  platforms  or  crib  work  are  sometimes  entirely 
satisfactory  when  placed  4  or  5  ft.  below  ground,  or  at  about 
one-half  the  penetration  of  the  pole.  These  platforms  serve  a 
triple  purpose  in  that  they  prevent  tilting,  settlement,  and 
uplift.  To  be  effective  against  tilting,  they  must,  of  course,  be 


170 


POLE  AND  TOWER  LINES 


securely  fastened  to  the  pole,  either  at  two  elevations  or  partly 
by  means  of  inclined  hanger  rods. 

Concrete  heel  and  toe  braces  have  been  advocated,  but  it 
seems  inconsistent  to  use  concrete  as  a  brace  for  timber,  unless 
it  extends  above  the  ground  line  as  a  protection  against  decay. 
If,  however,  an  envelope  of  concrete  extending  1  to  2  ft. 
above  and  below  the  ground  line  is  to  be  used,  it  would  be 
advisable  and  economical  to  form  the  braces  of  concrete.  In 
the  writer's  opinion  a  short  envelope  of  concrete  like  that  just 
mentioned  is  very  desirable  for  important  wood  pole  lines 
where  the  cost  of  handling  water  and  aggregates  is  not  prohib- 


FIG.  106. — A-frame  foundation  (semi-obsolete  type). 

itive.  This  design  has  been  used  very  little,  but  it  appears  to 
possess  considerable  merit,  as  it  protects  the  timber  from  attack 
by  grass  fires,  and  decay  at  the  most  exposed  point,  and  further- 
more increases  the  lateral  stability  of  the  pole  by  increasing  its 
diameter  where  the  earth  resistance  is  weakest. 

Poles  set  in  stout  well-hooped  headless  barrels  filled  with  sand 
and  gravel  have  been  used  with  success  in  peat  bogs,  while  a 
removable  barrel  form  has  also  been  used  to  deposit  sand  and 
gravel  as  back  filling.  The  permanent  barrel  forms  a  large  butt 
and  retains  the  filling  which  might  otherwise  work  away  from 
the  pole  through  the  surrounding  earth.  In  case  the  subsoil  is 


FOUNDATIONS 


171 


compact,  the  barrel  may  not  be  required,  sufficient  resistance 
being  supplied  by  the  back  filling. 

It  has  always  seemed  to  the  writer  that  shallow  foundations 
were  particularly  objectionable  for  flexible  frames  and  that 
instead  of  a  penetration  of  5  ft.,  not  less  than  6  to  7  ft.  should  be 
used.  When  the  ground  is  loosened  in  the  spring  or  fall,  either 
by  thawing  or  rains,  the  upper  1  or  2  ft.  are  of  relatively  little 
service  in  resisting  uplift  or  lateral  pressure. 


FIG.  107. — Tower  foundation,  con- 
crete-filled sleeve. 


FIG.  108. — Wide-base  tower 
foundation. 


Unless  the  bottom  of  an  earth  excavation  is  naturally  very 
firm,  about  6  in.  of  mixed  sand  and  gravel,  or  sand  and  broken 
stone  should  be  tamped  into  the  bottom  of  the  hole  to  form  a 
firm  bearing  for  the  anchor  plates.  The  earth  back-fill  above  the 
anchor  plates  should  be  tamped  in  thin  layers.  Concrete  back 
filling  is,  of  course,  preferable,  but  its  cost  may  be  considerable 
when  it  is  necessary  to  haul  water  and  cement  and  perhaps  the 
sand  and  gravel. 


172 


POLE  AND  TOWER  LINES 


It  must  be  admitted  that  exact  and  concise  rules  for  founda- 
tion design  are  not  available,  and  this  condition  has  lead  to  in- 
adequate and  also  to  very  extravagant  designs.  The  latter  may 
in  part  be  due  to  the  assumption  of  unnecessarily  severe  condi- 


FIG.   109. — Tower  foundation  in  rotten  rock. 


FIG.  110. — Tower  foundation  in  rock. 

tions  of  loading.  Thus  it  will  be  found  that  the  foundation  for 
a  small-base  steel  tower  or  pole  may  become  excessive  if  de- 
signed for  a  factor  of  safety  of  2.0  for  all  wires  broken  and  using 
as  a  resisting  force  only  the  weight  of  the  30°  cone  of  earth. 


FOUNDATIONS  173 

Transmission-line  experience  tends  to  show  that  the  actual 
resistance  of  average  ground  must  be  greater  than  that  given 
by  the  usual  formulas.  This  may  be  due  to  the  probability  that 
the  specified  maximum  load  occurs  when  the  ground  is  frozen 
or  partly  frozen  and  that  under  such  conditions  high  resisting 
stresses  are  developed. 

The  assumption  that  the  resistance  is  merely  that  of  the 
weight  of  the  earth  inclosed  within  the  inverted  cone  outlined 
by  the  angle  of  repose  of  the  material,  assumed  as  30°  or  even 
45°,  cannot  be  correct  for  compact  soil,  clay,  etc.  It  would 
seem  that  good  ground,  particularly  if  bound  together  by  roots, 
stones,  etc.,  must  have  an  appreciable  shearing  value. 

CONCRETE 

Cement. — The  production  of  Portland  cement  in  this  country 
is  now  so  standardized  that  any  brand  of  Portland  cement  will 
pass  the  standard  tests,  provided  the  cement  in  question  is  a 
representative  sample  of  the  maker's  output.  The  chief  purpose 
of  cement  specifications  is,  therefore,  to  secure  the  proper  quality 
rather  than  to  discriminate  between  brands.  It  should  not  be 
understood  that  all  cement  is  necessarily  satisfactory,  since  any 
particular  lot  may  have  been  injured  by  an  error  in  manufacture 
or  by  improper  or  overlong  storage.  Any  reputable  manufacturer 
can  and  does  make  a  satisfactory  cement  and  the  cement  tests 
should,  therefore,  be  used  merely  to  guard  against  error.  The 
manipulation  of  the  tests  and  the  requirements  to  be  met  by 
the  cement  have  been  very  completely  standardized  and  are 
given  in  what  are  known  as  the  "  Standard  Specifications  for 
Cement, "  copies  of  which  may  be  obtained  from  any  cement 
manufacturer  or  from  the  Association  of  Portland  Cement 
Manufacturers,  so  they  will  not  be  repeated  here. 

Proportions. — The  proportions  of  cement,  sand  and  stone,  or 
gravel,  will  depend  somewhat  on  the  purpose  for  which  the  con- 
crete is  to  be  used.  In  general,  the  smaller  the  volume  and  the 
greater  the  stresses,  the  richer  the  concrete  should  be.  Further, 
if  the  concrete  is  to  be  impervious  to  water  or  is  to  be  immersed 
in  water,  or  deposited  through  water,  the  mixture  should  be 
richer,  i.e.,  with  a  larger  amount  of  cement  than  would  otherwise 
be  necessary.  Concrete  is  stronger,  more  impervious,  and  perma- 
nent when  it  is  of  the  maximum  density.  The  maximum  density 


174  POLE  AND  TOWER  LINES 

obtainable  from  any  given  sand,  cement  and  stone,  or  gravel,  will 
be  that  due  to  one  certain  proportion  of  the  ingredients.  The 
proper  proportions  in  any  particular  case  will  be  determinable 
by  tests  designed  to  disclose  the  voids  in  the  aggregates.  Or- 
dinarily, however,  it  is  not  necessary  to  make  such  tests,  as  the 
customary  proportions,  combined  with  good  workmanship,  will 
produce  a  satisfactory  result.  The  amounts  of  sand  and  stone 
have  usually  been  given  separately,  although  in  reality  there 
should  be  two  proportions — that  of  the  cement  and  that  of  the 
combined  sand  and  stone.  The  most  commonly  used  pro- 
portions are:  1:6  (1:2:4)  for  fine  work,  and  1:9  (1:3:6)  for  mass 
foundations,  etc. 

Aggregates. — The  aggregates,  which  are  the  sand,  gravel, 
broken  stone,  slag,  cinders,  chats,  etc.,  may  be  of  various  sizes 
from  screenings  to  fairly  large  stones.  They  should,  however, 
be  of  graded  sizes  in  order  to  present  fewer  voids.  Inasmuch  as 
concrete  is  in  reality  an  artificial  stone,  its  constituent  parts  must 
be  free  from  vegetable  matter  and  soft  particles,  or  the  resulting 
product  will  be  in  the  nature  of  " rotten  rock."  It  is  frequently 
specified  that  the  sand  and  other  aggregates  shall  be  clean,  al- 
though a  small  percentage  of  clay  is  generally  permissible,  since 
neither  sand  nor  gravel  will  be  perfectly  clean  without  very 
thorough  washing.  It  has  also  been  required  that  the  broken 
stone  shall  be  sharp,  but  this  is  generally  not  necessary  since  a 
high  grade  of  concrete  can  be  made  with  gravel,  and  gravel  is 
never  sharp.  Sharp  sand,  however,  is  desirable. 

Water. — The  water  used  in  mixing  concrete  should  be  free 
from  oils,  acids,  or  any  considerable  amounts  of  alkali  or  vege- 
table matter.  Satisfactory  water  can  generally  be  obtained 
throughout  the  country,  and  usually  near  the  site  of  the  work. 
Although  "dry  concrete"  has  been  used  to  a  considerable  extent 
abroad,  and  was  formerly  used  somewhat  in  this  country,  the 
present  practice  here  is  to  use  "wet  concrete."  By  wet  concrete 
is  meant  concrete  mixed  with  sufficient  water  to  be  semi-fluid, 
so  that  it  may  readily  flow  around  the  reinforcing  or  incased 
material,  and  be  easily  tamped  or  puddled.  This  is  necessary 
to  completely  fill  the  forms,  and  obtain  an  efficient  adherence  to 
the  reinforcement.  The  only  objection  to  the  use  of  an  excess 
of  water  is  that  some  of  the  cement  will  be  washed  away  or  de- 
posited separately,  and  that  the  resulting  concrete  sets  and 
dries  more  slowly,  thus  delaying  the  work.  Since  water  is  needed 


FOUNDATIONS  175 

both  for  fluidity  and  chemical  combination  a  sufficient  quantity 
must  be  provided  to  prevent  its  absorption  by,  or  drying  on, 
the  aggregates.  In  warm  weather  particularly,  it  is  advisable 
to  thoroughly  "wet  down"  the  pile  of  stone  from  which  the  ma- 
terial is  taken. 

Mixing  and  Placing. — Concrete  may  be  mixed  either  by  hand 
or  by  mechanical  mixers,  the  method  in  any  instance  depending 
on  the  quantity  to  be  made  and  the  availability  of  a  mixer  at 
the  site.  Machine  mixing  is  probably  more  thorough  than  hand 
mixing,  although  just  as  good  concrete  can  be  made  by  hand 
under  proper  supervision.  In  hand  mixing,  the  materials  should 
be  mixed  on  a  flat  form  or  floor  to  prevent  an  excessive  loss  of 
cement-bearing  water,  or  the  admixture  of  earth,  etc.  Mixing 
floors  are  of  various  sizes  from  about  6  ft.  square  up  to  much  larger 
areas,  but  in  any  case  it  is  desirable  that  tongue-and-grooved 
lumber,  or  two  layers  of  lumber,  be  used  to  decrease  the  leak- 
age through  the  cracks. 

In  order  to  obtain  the  proper  proportions  of  the  aggregates 
some  unit  of  measurement  such  as  a  bucket  or  barrow  should 
be  used  to  transport  them  from  the  stock  piles  to  the  mixing 
platform.  It  is  then  a  simple  matter  for  the  workmen  to  regu- 
larly take  so  many  units  of  sand,  another  number  of  units  of 
stone  and  the  specified  number  of  bags  of  cement  to  make  each 
batch  of  concrete.  The  sand  and  stone  are  first  placed  on  the 
mixing  board  and  mixed  by  turning  the  pile  over  with  shovels. 
The  cement  is  then  spread  over  the  mass  which  is  turned,  water 
being  added  during  the  turning.  The  number  of  turns  to  be 
given  each  process,  and  the  faithfulness  with  which  the  work  is 
done  determines  the  excellence  of  the  mixing.  Mixing  should 
be  continued  until  the  mass  presents  a  uniform  appearance,  and 
the  stone  appears  to  be  entirely  covered  with  sand  and  cement. 
As  soon  as  the  mixing  is  completed,  the  material  should  be  taken 
in  water-tight  buckets  or  barrows,  and  placed  in  the  work.  The 
size  of  the  batch  should  depend  on  the  amount  that  can  be  im- 
mediately used,  since  material  left  on  the  board  for  any  consider- 
able time  takes  an  initial  set,  and  is  useless  for  future  work. 

The  placing  of  concrete  should  be  as  nearly  continuous  as 
practicable  to  prevent  the  formation  of  cleavage  planes  or 
planes  having  little  cohesion.  Such  surfaces  will  contain  a 
layer  of  "  dead"  material  as  well  as  a  certain  amount  of  dirt  which 
floats  to  the  surface.  In  poor  grades  of  work  the  joints  can  be 


176  POLE  AND  TOWER  LINES 

distinctly  seen  on  the  sides  of  the  structure.  Since  it  is  not 
always  possible  to  work  continuously,  the  temporary  surface 
should  be  left  rough  and  should  be  thoroughly  washed  and 
preferably  scrubbed  before  continuing  operations.  Reinforced- 
concrete  poles  should  always  be  made  in  one  operation  as  they 
are  too  small  to  justify  the  risks  attending  non-continuous  work. 

Forms. — The  finish  and  general  excellence  of  the  forms  should 
depend  on  the  character  and  magnitude  of  the  work.  In  case  a 
ground  stub  or  the  base  of  a  wood  or  steel  pole  is  to  be  incased 
in  concrete,  no  forms  will  be  necessary  as  the  earth  will  serve  the 
same  purpose.  A  small  collar  may  be  used,  however,  at  the 
ground  level  to  retain  the  concrete  above  the  ground.  Forms 
are  generally  of  wood,  but  in  case  there  is  considerable  repetition 
in  the  construction,  it  will  often  prove  economical  to  use  metal, 
as  metal  forms  last  longer,  retain  their  shape,  are  more  readily 
cleaned  and  produce  a  smoother  finish  on  the  surface  of  the 
concrete. 

The  thickness  of  the  lumber  and  the  amount  of  bracing 
necessary  for  wooden  forms  will  depend  on  the  size  and  shape 
of  the  structure.  Where  the  same  form  is  to  be  used  again  and 
again,  it  will  prove  economical  to  use  thicker  material  and  better 
made  forms  than  for  a  single  unit,  since  thin  or  flimsy  forms  be- 
come warped  and  broken  in  handling.  The  inner  surface  of  the 
form  should  be  of  faced  lumber  and  fillets  should  be  placed  in 
the  corners,  since  sharp  edges  are  difficult  to  make  and  rarely 
permanent.  On  pole  foundations  the  corners  above  ground 
should  be  beveled  or  worked  into  a  curve. 

The  proper  time  for  the  removal  of  forms  will  vary  considerably, 
depending  on  the  temperature  and  the  kind  of  cement.  Concrete 
sets  slowly  in  cold  weather  and  also  under  certain  conditions  of 
the  atmosphere  may  take  a  much  longer  period  than  is  usually 
required.  This  delayed  set  is  not  due  to  freezing  or  very  cold 
weather,  and  its  exact  nature  is  not  well  understood.  There 
seems  to  be  nothing  to  do  but  to  wait  until  some  slight  change  in 
the  temperature  and  humidity  will  allow  the  concrete  to  take  its 
usual  set. 

In  order  to  prevent  the  adherence  of  concrete  to  the  forms  with 
the  consequent  danger  of  breaking  out  pockets  on  the  surface 
when  the  forms  are  removed,  they  may  be  coated  on  the  in- 
terior with  soap,  grease,  etc.  After  the  forms  are  removed  and  in 
some  cases  even  before  removal,  the  concrete  should  be  kept 


FOUNDATIONS  177 

damp  by  sprinkling.  This  is  particularly  desirable  in  warm 
weather  to  prevent  the  formation  of  surface  cracks. 
'  Workmanship. — As  indicated  above  the  character  of  the 
workmanship  may  properly  vary  for  different  kinds  of  con- 
struction, that  for  simple  and  massive  foundations  being  of 
a  lower  grade  than  would  be  necessary  for  reinforced  concrete 
in  small  volume  particularly  such  work  as  reinforced-concrete 
poles.  Poles  require  the  very  best  possible  grade  of  workmanship 
and  material,  as  well  as  competent  designing  and  supervision. 

It  is  not  always  possible  to  avoid  concrete  construction  in 
cold  weather,  but  if  the  temperature  is  much  below  freezing  or 
even  slightly  below  for  continued  periods,  it  is  necessary  to  adopt 
some  method  to  prevent  the  concrete  from  freezing  before  it 
has  set.  While  frozen  concrete  will  usually  set  after  it  has  thawed, 
there  is  a  possibility  of  its  failing  if  loads  are  applied  while  it  is 
still  frozen.  When  the  temperature  during  the  day  is  above 
freezing,  even  if  the  nights  are  cold,  the  heat  generated  by  the 
concrete,  will  often  be  sufficient  to  prevent  freezing.  Otherwise, 
it  becomes  necessary  to  heat  the  sand  and  stone  before  mixing 
and  to  enclose  the  concrete  with  planks,  canvas,  straw,  etc.,  or 
to  heat  the  enclosure  by  fires  or  steam. 

Forms  serving  as  temporary  supports  for  the  concrete  should 
not  be  removed  too  promptly.  Pole  and  tower  foundations, 
however,  may  have  their  forms  removed  rather  quickly,  since 
the  forms  do  not  usually  serve  to  support  the  concrete.  The 
early  removal  of  forms  is  often  due  to  the  desire  to  rub  the  exposed 
surfaces  before  the  concrete  has  become  very  hard.  It  is  entirely 
possible,  however,  with  the  use  of,  perhaps,  a  little  additional 
energy,  to  rub  to  a  smooth  finish  surfaces  which  have  set  for 
several  weeks. 

It  is  generally  specified  that  surfaces  shall  not  be  finished  by 
plastering  on  a  coating  of  cement  mortar,  but  this  requirement  is 
not  always  properly  enforced.  Since  such  plastered  material 
will  frequently  spall  off,  it  is  far  better  to  rub  the  surface  with  a 
concrete  brick  or  carborundum  stone,  using  water  instead  of 
mortar.  By  this  means,  the  marks  of  the  forms  as  well  as  an 
outer  layer  of  cement  are  removed  leaving  a  permanent  sand- 
stonelike  finish. 

Reinforcement. — Although  some  specifications,  notably  those 
of  the  Joint  Committee  on  Concrete  and  Reinforced  Concrete, 

have  permitted  only  medium-grade  steel,  there  appears  to  be 
12 


178  POLE  AND  TOWER  LINES 

absolutely  no  necessity  of  adhering  to  this  requirement  in  trans- 
mission-line construction  provided  high  carbon  steels  are  used 
with  proper  design  and  inspection.  Open-hearth  high-carbon 
steel  rods  are  obtainable,  as  well  as  those  of  rerolled  rail  stock, 
with  an  ultimate  strength  of  80,000  to  100,000  Ib.  per  square 
inch,  and  an  elastic  limit  of  40,000  to  60,000  Ib.  per  square  inch, 
and  which  are  capable  of  being  bent  cold  without  fracture  about 
a  radius  equal  to  four  times  the  diameter  of  the  rod. 

Such  material  cannot  be  subjected  to  the  same  amount  of 
abuse  in  handling  as  the  low-carbon  steels,  and  care  is  necessary 
in  testing  and  inspection  to  insure  that  any  given  lot  of  rods  will 
have  about  the  same  characteristics.  When  intelligently  used, 
however,  such  material  is  entirely  satisfactory  for  certain  classes 
of  transmission-line  construction. 

The  system  of  reinforcement  should  provide  material  designed 
to  prevent  or  at  least  minimize  cracks  due  to  shrinkage.  In 
general  this  is  accomplished  by  the  use  of  both  longitudinal  and 
transverse  reinforcing  metal.  While  in  some  instances  shrink- 
age cracks  may  not  impair  the  strength  of  the  construction  they 
might,  by  the  admission  of  water,  cause  corrosion  of  the  reinforc- 
ing metal. 

The  reinforcement  should  not  be  composed  of  a  few  large  bars 
when  it  is  possible  to  use  smaller  rods  without  a  material  reduc- 
tion in  the  effective  lever  arm.  Very  small  rods  are  not  always 
desirable,  but  the  use  of  two  medium-sized  rods  in  place  of  one 
very  large  one  allows  greater  uniformity,  and  a  maximum 
adherence  of  the  concrete  to  the  skeleton. 

Waterproofing,  Salt  Water,  Alkali,  Etc. — Waterproofing  of 
transmission-line  structures  will  usually  be  confined  to  the  ex- 
clusion of  water  from  the  incased  reinforcing  rods  or  structural 
steel.  It  seems  probable  that  the  most  convenient  form  of  pro- 
tection will  be  a  dense  mixture  and  a  firm  uncracked  surface. 
In  other  classes  of  construction,  waterproofing  by  the  various 
tar  and  felt  processes  is  efficient  when  well  done,  and  in  some  cases 
the  addition  of  waterproofing  compounds  to  the  cement  has  given 
satisfactory  service.  In  general,  however,  the  same  amount  of 
care  if  expended  in  making  proper  concrete  will  presumably  pro- 
vide equal  protection  to  line  structures.  The  thickness  of  con- 
crete outside  any  rod  or  embedded  metal  must  be  sufficient  to 
minimize  the  admission  of  water  through  hair  cracks  or  by  capil- 
lary action.  In  concrete  poles  the  outer  thickness  of  concrete 


FOUNDATIONS  179 

will  rarely  exceed  1%  m-  whereas  in  other  construction  6  in.  can 
be  given  at  little  additional  expense. 

Concrete  which  is  to  be  immersed  in  sea  water  must  be  made 
of  richer  mixtures  than  otherwise  required  and  care  is  essential 
to  secure  a  dense  mixture  and  an  impervious  surface.  If  the 
concrete  can  be  allowed  to  become  hard  before  the  sea  water  is 
permitted  to  come  into  contact  with  it,  its  resistance  to  disinte- 
gration will  be  increased.  In  many  instances  the  disintegration 
of  concrete  exposed  to  sea  water  has  unquestionably  been  due  in 
part  to  mechanical  abrasion  from  floating  ice,  logs,  etc.  Added  to 
this  there  has  been  a  minute  spalling  of  the  surface  due  to  freez- 
ing in  the  surface.  The  chemical  attack  by  sea  water  is  believed 
to  be  caused  by  the  replacement  of  a  part  of  the  CaO  of  the 
cement  by  MgO  from  the  sea  water,  and  also  by  a  change  in 
the  proportions  of  silica  and  SO3.  The  most  effective  protection 
against  sea  water  is  believed  to  be  provided  by  a  properly  pro- 
portioned very  dense  concrete,  rather  than  by  the  addition  of  any 
of  the  various  materials  which  are  intended  to  reduce  permeability. 

In  some  sections  of  the  country,  particularly  in  certain  western 
states,  the  soil  or  ground  water  may  contain  sufficient  acid  or 
alkali  to  affect  concrete  unless  the  concrete  is  carefully  made. 
As  in  the  case  of  waterproofing  or  salt  water  concrete,  the  most 
effective  protection  against  acid  or  alkali  is  dense  concrete,  al- 
though this  should  be  supplemented  by  the  provision  of  aggre- 
gates inert  to  the  acids  or  alkalis.  If  cinders  are  used  as  an  aggre- 
gate, they  should  not  contain  much  sulphur,  and  should  be  hard 
and  fairly  non-porous. 

Accurate  and  complete  data  are  lacking  in  relation  to  the 
service  of  concrete  subjected  to  electric  currents,  but  it  is  probable 
that  an  embedded  rod  which  acts  as  the  anode  of  an  electric 
circuit  is  liable  to  corrode  and  to  disrupt  the  concrete  in  contact 
with  it.  Since  corrosion  caused  by  electrolysis  is  confined  to  the 
surface  from  which  a  current  flows,  it  would  appear  practicable 
to  remove  any  liability  to  disintegration  in  line  structures  by 
grounding  them  and,  perhaps,  by  allowing  the  incased  material 
to  project  through  the  bottom  of  the  concrete  into  the  earth. 


CHAPTER  X 
PROTECTIVE  COATINGS 

Paint  and  Painting. — The  careless  application  of  paint  and  the 
use  of  inferior  grades  of  paint  on  transmission  line  structures 
cannot  be  too  strongly  condemned.  Although  paints  range  in 
price  from  about  40  cts.  to  $1.40  per  gallon,  the  possible  saving 
afforded  by  using  the  lower  priced  paints  is  small  compared  with 
the  risk  of  deterioration.  The  usual  requirements  for  painting 
are  few  in  number  and  impossible  of  misinterpretation,  but  un- 
fortunately they  are  usually  more  or  less  disregarded. 

Inasmuch  as  the  adherence  of  the  first  coat  of  paint,  as  well  as 
all  subsequent  coats,  depends  on  the  manner  in  which  the  first 
coat  is  applied  and  on  the  character  of  the  surface  to  which  it 
is  applied,  inspection  should  begin  with  the  shop  coat.  Careful 
inspection  of  such  work  is,  however,  the  exception  rather  than 
the  rule.  All  painting  specifications  require  that  the  surfaces 
shall  be  thoroughly  cleaned  of  all  mill  scale,  rust  and  dirt,  and  that 
no  painting  shall  be  done  in  the  rain.  It  is  to  be  inferred  that 
exposing  fresh  paint  to  rain  and  cinders  from  a  railroad  or  ship- 
ping yard  would  be  equally  undesirable.  Unfortunately,  most 
manufacturers  have  but  meager  covered-storage  facilities  and  a 
protracted  period  of  rain,  or  insistence  on  the  part  of  purchasers 
that  shipments  be  made  immediately,  will  often  result  in  undue 
exposure  of  freshly  painted  structures.  Further,  the  application 
of  the  second  or  field  coat  is  frequently  open  to  improvement, 
in  that  careless  workmen  will  paint  over  mud  spots  or  blisters. 
Since  the  top  coat  has  no  adherence  to  the  metal  at  such  points, 
it  will  flake  off  when  the  mud  loosens  or  the  blister  cracks.  If 
the  shop  coat  is  in  bad  condition  in  any  place  it  should  be  scraped 
clean  and  repainted.  Some  muscular  effort  is  necessary  to  a 
proper  application  of  paint  and  the  cost  of  worn-out  brushes  will 
be  amply  repaid  by  the  ultimate  result. 

The  use  of  thinners  in  general  or  the  addition  of  benzine  to 
ready-mixed  paints  may  effect  a  little  saving  in  the  initial  cost 
of  labor  and  material,  but  will  not  be  economical  in  final  cost. 

180 


PROTECTIVE  COATINGS  181 

The  field  coat  should  be  applied  after  the  wires  have  been  strung 
so  that  the  final  coat  will  not  be  marred  by  erection  operations. 
Sometimes  one  shop  coat  and  two  field  coats  are  applied,  but  it 
seems  that  one  of  these  coats  might  preferably  be  postponed  for 
a  few  years,  and  a  portion  of  its  cost  utilized  in  improving  the 
initial  painting. 

It  is  the  writer's  belief  that  for  structures  with  small  or  medium- 
spread  bases  reasonably  thick  painted  material  will  usually  prove 
more  satisfactory  and  economical  than  thin  galvanized  sections, 
provided  subsequent  maintenance  is  not  grossly  neglected.  As 
transmission  line  supports  are  widely  scattered,  it  has  not  been 
customary  for  the  linemen  or  patrol  men  to  pay  much  attention 
to  the  physical  condition  of  metal  structures,  with  the  result  that 
maintenance  painting  has  been  postponed  until  quite  noticeable 
rusting  has  taken  place. 

The  use  of  a  shop  coat  of  linseed  oil  instead  of  paint,  although 
not  uncommon  some  years  ago  in  bridge  work,  is  rarely  found  in 
transmission  line  construction,  nor  does  there  appear  any  good 
reason  for  its  use.  Such  coats  unless  carefully  applied  are  very 
liable  to  blister  and  peel  either  before  or  after  the  field  coat  is 
applied.  Further,  such  primers  add  nothing  to  the  adherence 
of  the  final  coats,  and  are  of  far  less  service  than  even  a  mediocre 
shop  coat. 

In  repainting,  i.e.,  maintenance  painting,  just  as  much,  indeed 
more,  care  is  needed  than  in  the  original  applications.  This  is  due 
to  the  fact  that  in  addition  to  removing  dirt,  etc.,  it  is  necessary 
to  remove  paint  if  rusting  has  occurred  beneath  it,  since  the  new 
covering  of  paint  will  not  entirely  prevent  the  continued  action 
of  the  rust.  As  in  the  original  painting,  no  efficient  service  need 
be  expected  of  paint  applied  to  a  wet  surface. 

All  paints  should  be  stirred  before  using,  particularly  the  heavy 
pigment  paints  such  as  red  lead.  A  good  rule  is  to  require  the 
painters  to  stir  the  barrel  thoroughly  before  replenishing  any 
individual  can.  Barrels  which  have  been  opened  should  be  kept, 
not  only  under  cover,  but  covered,  otherwise  water,  dirt  and  other 
substances  become  admixed  with  the  paint,  or  else  the  paint 
becomes  dried  out,  thick  and  dead.  The  writer  is  inclined  to 
heartily  recommend  a  plaster  of  "paint  skins"  about  the  point 
of  entrance  of  the  superstructure  into  the  concrete,  but  no 
one  would  advocate  an  attempt  to  spread  such  a  thick  mortar-like 
substance  over  the  large  areas  of  the  superstructure.  Even 


182 


POLE  AND  TOWER  LINES 


under  rather  careful  supervision,  the  bottom  of  the  barrel  is  a 
thicker  more  heavily  pigmented  paint  than  the  half  first  used. 
If  to  the  thick  sediment  remaining  in  the  lowest  quarter  barrel — due 
perhaps  in  part  to  evaporation — benzine  be  added,  the  result  can- 
not in  justice  to  the  manufacturer  be  termed  an  equivalent  paint. 
In  general,  the  brand  of  paint  or  type  of  pigment  is  of  far  less 
importance  in  the  ultimate  result  than  a  little  care  and  intelli- 
gence in  its  application.  To  go  further,  even  the  " intelligence" 
might  be  omitted  since  ignorance  should  be  able  to  rub  dirt  and 
rust  off  and  rub  paint  on. 

CONDITION  OP  PAINTED  STRUCTURES  AS  REPORTED  IN  1915  BY  Six  LINES 
HAVING  A  TOTAL  OP  1686  TOWERS  IN  SERVICE 


Number 
of 
structures 

Year 
built 

Foundation 

Condition  as  reported  in  1915 

697 

1909 

Concrete 

Fine  condition,  have  not  been  repainted. 

127 

1911 

Galvanized 

Very  good,  slight  rust  under  paint  in  spots, 

due  to  careless  cleaning. 

78 

1912 

Concrete 

Slight  corrosion. 

364 

1912 

Concrete 

Corrosion,  only  a  shop  coat  was  put  on  and 

rusting   occurred  before  first  field   coat 

was  added  in  1914. 

262 

1913 

Very  little  corrosion. 

158 

1913 

Galvanized 

No  corrosion. 

1686 

' 

Galvanizing. — Under  favorable  climatic  conditions,  particu- 
larly over  rough  unsettled  country  where  all  maintenance  opera- 
tions are  expensive,  galvanizing  is  thought  to  be  more  economical 
than  painting,  at  least  for  wide  base  towers.  In  the  more  com- 
pact structures  the  cost  of  painting  is  reduced  owing  to  the  ease 
with  which  a  painter  can  move  about.  No  very  definite  knowl- 
edge exists  as  to  the  regions  in  which  galvanizing  does  not  afford 
proper  protection,  although  it  is  generally  assumed  as  being  un- 
satisfactory in  the  neighborhood  of  coke  ovens,  smelters,  steam 
plants,  etc.,  and  near  the  seacoast.  It  may  also  be  true  that 
certain  soils  such  as  swamps,  induce  rapid  deterioration.  It 
should  be  noted  that  the  use  of  sections  J^  in.  in  thickness  pre- 
supposes that  their  protection  by  galvanizing  will  be  absolutely 
effective.  The  galvanizing  of  structural  members — by  the  hot 
process — can  be  done  in  a  very  uniform  manner  and  in  strict 


PROTECTIVE  COATINGS 


183 


accordance  with  the  standard  requirement,  even  though  the 
real  efficiency  of  the  average  inspection  is  open  to  question. 
The  standard  test  is  an  accelerated  specimen  test,  and  although 
the  only  practicable  one,  is  subject  to  the  usual  criticisms  of 
such  tests. 

CONDITION  OP  GALVANIZED  STRUCTURES  AS  REPORTED  IN  1915  BY  NINETEEN 
LINES  HAVING  A  TOTAL  OP  9269  TOWERS  IN  SERVICE 


Number 
of 
structures 

Year 
built 

Foundation 

Condition  as  reported  in  1915 

184 

1906 

Earth 

Good  condition. 

160 

1906 

Earth 

Fair  condition. 

111 

1909 

Earth 

No  corrosion. 

244 

1909 

Earth 

Good  condition. 

913 

1909 

Painted 

Some  corrosion  when  exposed  to  salt  spray 

and  salt  fogs. 

220 

1911 

Earth 

No    particular    signs    of    rust,    except    on 

sherardized  bolts. 

242 

1911 

Earth 

No  corrosion. 

82 

1911 

Concrete 

Entirely  satisfactory,  last  inspection. 

378 

1911 

Concrete 

Good  condition. 

748 

1911 

Concrete 

Good  condition. 

593 

1912 

Earth 

No  signs  of  corrosion. 

1041 

1912 

Concrete 

No  signs  of  corrosion. 

324 

1912 

Painted 

Unimpaired. 

1852 

1912- 

Earth 

Good  condition. 

1914 

1079 

1913 

Earth 

Good  condition. 

33 

1913 

Concrete 

No  corrosion. 

851 

1913 

Concrete 

No  corrosion. 

64 

1913 

Concrete 

No  corrosion. 

150 

1913 

Earth 

No  particular  signs  of  rust,  except  sherar- 

dized bolts. 

9269 

The  ideal  galvanizing  consists  in  a  thick,  tough  layer  of  zinc 
absolutely  free  from  pin  holes  and  adhering  firmly  to  the  steel. 
A  pure  high-grade  coating  of  zinc  is  more  likely  to  be  tough  and 
to  have  a  good  adherence  to  the  steel  than  one  which  is  composed 
of  lower  grade  material.  The  excellence  of  the  pickling  and 
cleaning  will  have  a  marked  effect  upon  the  existence  of  pin 
holes,  since  these  are  generally  caused  by  more  or  less  minute 
specks  of  scale,  etc.  The  efficiency  of  the  final  work  may,  there- 


184  POLE  AND  TOWER  LINES 

fore,  depend  either  on  the  material  of  the  bath  or  on  the  work- 
manship. 

In  order  to  obtain  adherence,  the  steel  is  first  pickled  in  a 
weak  solution  of  sulphuric  acid  to  expose  a  clean  surface.  The 
pickling  may  or  may  not  be  followed  by  washing,  but  it  should 
be  followed  by  an  examination  and  removal  of  spots.  The  next 
step  is  the  muriatic  acid  bath  after  which  the  metal  is  heated 
either  in  an  oven  or  by  placing  it  over  the  bath  of  hot  zinc. 

The  material  to-  be  coated  is  then  immersed  in  the  hot  bath  of 
molten  zinc,  and  allowed  to  remain  therein  until  in  the  judgment 
of  the  foreman  the  covering  is  complete.  Before  entirely  re- 
moving the  material  from  the  bath  it  is  allowed  to  drain  to 
recover  the  excess  zinc.  This  process  is  the  cause  of  the  small 
projections,  or  icicles,  of  zinc  which  are  frequently  to  be  found 
on  galvanized  structural  material.  Apart  from  the  question  of 
appearance,  they  do  no  harm,  unless  at  contact  surfaces,  and 
are  to  some  extent  at  least  the  sign  of  a  thick  bath. 

The  entrance  end  of  the  zinc  bath  contains  a  floating  bath  of 
sal  ammoniac  retained  in  position  by  movable  plates  set  verti- 
cally in  the  bath.  All  entering  material  is  inserted  through  this 
"flux." 

The  exit  end  of  the  zinc  bath  or  •"  kettle"  may  contain  an 
admixture  of  " temper  metal,"  added  for  the  purpose  of  increas- 
ing the  luster  and  insuring  a  more  fluid  bath.  This  added 
material  contains  various  proportions  of  tin,  aluminum  and 
spelter.  The  addition  of  tin  produces  the  spangled  appearance 
used  for  galvanized  buckets,  boilers,  etc.,  and  this  ingredient  will 
generally  be  found  in  the  temper  metal  of  baths  used  for  general 
commercial  galvanizing.  The  amount  of  tin,  however,  should  be 
small,  and  if  used  it  should  be  placed  only  in  the  exit  end  of  the 
bath,  since  an  excessive  quantity,  particularly  in  the  inside  layer, 
may  result  in  the  formation  of  a  more  brittle  coating. 

If  there  is  any  mill  scale  on  the  black  material,  which  is  not 
loosened  and  removed  in  the  pickle  or  before  galvanizing,  it  will 
result  in  a  blotch  of  spelter  which  may  or  may  not  become  loos- 
ened subsequently. 

Owing  to  the  restricted  size  of  the  hot  bath,  although  some  of 
them  are  about  3  ft.  square  and  25  ft.  long,  it  is  necessary  to  dip 
long  members  from  each  end.  This  often  results  in  a  lap  or 
wave  in  the  coating  at  the  middle  of  the  piece,  which  is  not  in 
itself  objectionable.  The  chief  criticism  of  double  end  dipping 


PROTECTIVE  COATINGS  185 

is  that  it  often  causes  warping  of  certain  kinds  of  material  such 
as  wide  plates  or  thin  flexible  pieces. 

The  spelter  to  be  used  in  galvanizing  wire  should  be  that  known 
as  High  Grade,  and  the  spelter  for  galvanizing  structural  shapes 
should  be  Prime  Western,  or  equal. 

It  has  frequently  been  claimed  that  paint  will  not  adhere  to  a 
galvanized  coat.  Whatever  the  fact  with  respect  to  subsequent 
painting  with  structural  paints,  there  is  no  difficulty  in  painting 
the  assembling  and  shipping  marks  on  the  finished  members 
with  some  interior  paints. 

Galvanizing  is  one  of  the  few  operations  in  which  there  has 
been  no  considerable  attempt  either  to  develop  complete  methods 
of  inspection,  or  to  make  any  inspection  of  the  spelter  material 
as  such.  In  view  of  the  different  qualities  of  spelter  obtainable 
it  would  seem  advisable  to  periodically  take  samples  from  the 
middle  of  the  bath  and  subject  "them  to  a  chemical  analysis. 
The  following  material  specification  represents  fair  commercial 
grades  of  spelter  suitable  for  wire  and  structural  galvanizing. 

ABSTRACT  FROM  STANDARD  SPECIFICATIONS  FOR  SPELTER 
American  Society  for  Testing  Materials.    Adopted  August  21,  1911 

Under  these  specifications  Virgin  Spelter,  that  is,  spelter  made  from 
ore  or  similar  raw  material  by  a  process  of  reduction  and  distillation 
and  not  produced  from  reworked  metal,  is  considered. 

A HIGH  GRADE. 

D '..-.   PRIME  WESTERN. 

A  brand  shall  be  cast  in  each  slab  by  which  the  maker  and  grade  can 
be  identified. 

The  maker  shall  use  care  to  have  each  carload  of  as  uniform  quality  as 
possible. 

A.  HIGH  GRADE. — The  spelter  shall  not  contain  over 

0.07  per  cent.  lead. 
0.03  per  cent.  iron. 
0.05  per  cent,  cadmium. 

It  shall  be  free  from  aluminum. 

The  sum  of  the  lead,  iron  and  cadmium  shall  not  exceed  0.10  per  cent. 

D.  PRIME  WESTERN. — The  spelter  shall  not  contain  over 


1.50  per  cent.  lead. 
0.08  per  cent.  iron. 


186  POLE  AND  TOWER  LINES 

The  slabs  shall  be  reasonably  free  from  surface  corrosion  or  adhering 
foreign  matter. 

No  less  than  ten  slabs  shall  be  taken  as  a  sample  from  each  car;  for 
smaller  lots,  in  the  same  proportion  to  the  total  number,  but  in  no  case 
less  than  three  slabs.  In  case  of  dispute  half  of  the  sample  is  to  be 
taken  by  the  maker  and  half  by  the  purchaser;  and  the  whole  shall  be 
mixed. 

The  slabs  selected  as  samples  are  to  be  sawed  completely  across  and 
the  sawdust  used  as  a  sample.  In  case  no  saw  is  available  for  this  pur- 
pose, the  slabs  should  be  drilled  completely  through  and  the  drillings 
cut  up  into  short  lengths.  The  saw  or  drill  used  must  be  thoroughly 
cleaned.  No  lubricant  shall  be  used  in  either  sawing  or  drilling  and  the 
sawdust  or  drilling  must  be  carefully  treated  with  a  magnet  to  remove 
any  particles  of  iron  derived  from  the  tools. 

LEAD. — For  the  determination  of  lead  in  High  Grade  not  less  than  25 
grams,  and  in  Prime  Western  not  less  than  5  grams  shall  be  taken;  that 
is,  the  sample  used  for  analysis  should  not  contain  less  than  0.01  gram 
lead. 

IRON. — The  sample  for  iron  should  contain  not  less  than  25  grams  for 
the  three  higher  grades  and  not  less  than  10  grams  for  Prime  Western. 
The  entire  sample  must  be  dissolved,  the  iron  precipitated  as  ferric- 
hydroxide,  then  redissolved,  reduced,  and  the  iron  determined  by 
titration. 

CADMIUM. — Dissolve  25  grams  in  330  cc.  of  a  solution  of  one  part  of 
hydrochloric  acid  (specific  gravity  1.2)  and  five  parts  of  water.  Let  it 
stand  over  night;  filter  and  wash;  reject  filtrate  and  dissolve  the  residue, 
which  should  be  about  5  per  cent,  of  the  zinc,  in  nitric  acid.  Add  10  cc. 
of  sulphuric  acid;  evaporate  to  fumes;  dilute  and  filter  out  and  wash  the 
lead  sulphate.  Dilute  the  solution  to  500  cc. ;  add  5  grams  of  ammonium 
chloride;  pass  a  slow  stream  of  hydrogen  sulphide  for  one  hour  and  let 
stand  for  about  five  hours;  filter,  wash  with  hot  water,  dissolve  in  10  cc. 
of  sulphuric  acid  and  50  cc.  of  water;  filter  and  wash.  Dilute  to  400  cc. ; 
precipitate  with  hydrogen  sulphide  as  before.  Weigh  as  cadmium 
sulphide  or  dissolve  in  hydrochloric  acid  and  titrate  with  potassium 
ferrocyanide. 

The  following  standard  specifications  for  galvanizing  are  those 
now  adopted  by  practically  all  interested  parties  and  are  applic- 
able to  all  galvanized  material. 

STANDARD  SPECIFICATIONS  FOR  GALVANIZING 

(1911  Edition) 

These  specifications  give  in  detail  the  test  to  be  applied  to  galvanized 
material.  All  specimens  shall  be  capable  of  withstanding  these  tests. 


PROTECTIVE  COATINGS  187 

A.  COATING. — The  galvanizing  shall  consist  of  a  continuous  coating 
of  pure  zinc  of  uniform  thickness  and  so  applied  that  it  adheres  firmly 
to  the  surface  of  the  iron  or  steel.    The  finished  product  shall  be 
smooth. 

B.  CLEANING. — The  samples  shall  be  cleaned  before  testing,  first 
with  carbona,  benzine  or  turpentine  and   cotton   waste  (not  with  a 
brush)  and  then  thoroughly  rinsed  in  clean  water  and  wiped  dry  with 
clean  cotton  waste. 

The  samples  shall  be  clean  and  dry  before  each  immersion  in  the 
solution. 

C.  SOLUTION. — The  standard    solution    of    copper    sulphate    shall 
consist  of  commercial  copper  sulphate  crystals  dissolved  in  cold  water, 
about  in  the  proportion  of  36  parts,  by  weight,  of  crystals  to  100  parts,  by 
weight,  of  water.     The  solution  shall  be  neutralized  by  the  addition  of 
an  excess  of  chemically  pure  cupric  oxide  (CuO).     The  presence  of  an 
excess  of  cupric  oxide  will  be  shown  by  the  sediment  of  this  reagent  at 
the  bottom  of  the  containing  vessel. 

The  neutralized  solution  shall  be  filtered  before  using  by  passing 
through  filter  paper.  The  filtered  solution  shall  have  a  specific  gravity 
of  1.186  at  65°F.  (reading  the  scale  at  the  level  of  the  solution)  at  the 
beginning  of  each  test.  In  case  the  filtered  solution  is  high  in  specific 
gravity,  clean  water  shall  be  added  to  reduce  the  specific  gravity  to 
1.186  at  65°F.  In  case  the  filtered  solution  is  low  in  specific  gravity, 
filtered  solution  of  a  higher  specific  gravity  shall  be  added  to  make  the 
specific  gravity  1.186  at  65°F. 

As  soon  as  the  stronger  solution  is  taken  from  the  vessel  containing 
the  unfiltered  neutralized  stock  solution,  additional  crystals  and  water 
must  be  added  to  the  stock  solution.  An  excess  of  cupric  oxide  shall 
always  be  kept  in  the  unfiltered  stock  solution. 

D.  QUANTITY  OF  SOLUTION.- — Wire  samples  shall  be  tested  in  a  glass 
jar  of  at  least  two  (2)  in.  inside  diameter.     The  jar  without  the  wire 
samples  shall  be  filled  with  standard  solution  to  a  depth  of  at  least  four 
(4)  in.     Hardware  samples  shall  be  tested  in  a  glass  or  earthenware  jar 
containing  at  least  one-half  (y^  pt.  of  standard  solution  for  each 
hardware  sample. 

Solution  shall  not  be  used  for  more  than  one  series  of  four  immersions. 

E.  SAMPLES. — Not  more  than  seven  wires  shall  be  simultaneously 
immersed  and  not  more  than  one  sample  of  galvanized  material  other 
than  wire  shall  be  immersed  in  the  specified  quantity  of  solution. 

The  samples  shall  not  be  grouped  or  twisted  together,  but  shall  be  well 
separated  so  as  to  permit  the  action  of  the  solution  to  be  uniform  upon  all 
immersed  portions  of  the  samples. 

F.  TEST. — Clean  and  dry  samples  shall  be  immersed  in  the  required 
quantity  of  standard  solution  in  accordance -with  the  following  cycle  of 
immersions. 


188  POLE  AND  TOWER  LINES 

The  temperature  of  the  solution  shall  be  maintained  between  62° 
and  68°F.  at  all  times  during  the  following  test. 

First.     Immerse  for  one  minute,  wash  and  wipe  dry. 

Second.     Immerse  for  one  minute,  wash  and  wipe  dry. 

Third.     Immerse  for  one  minute,  wash  and  wipe  dry. 

Fourth.     Immerse  for  one  minute,  wash  and  wipe  dry. 

After  each  immersion  the  samples  shall  be  immediately  washed  in 
clean  water  having  a  temperature  between  62°  and  68°F.  and  wiped 
dry  with  cotton  waste. 

In  the  case  of  No.  14  galvanized  iron  or  steel  wire,  the  time  of  the 
fourth  immersion  shall  be  reduced  to  one-half  minute. 

G.  REJECTION. — If  after  the  test  described  in  Section  "F"  there 
should  be  a  bright  metallic  copper  deposit  on  the  samples,  the  lot 
represented  by  the  sample  shall  be  rejected. 

Copper  deposits  on  zinc  or  within  1  in.  of  the  cut  end  shall  not  be 
considered  causes  for  rejection. 

In  the  case  of  a  failure  of  only  one  wire  in  a  group  of  seven  wires 
immersed  together,  or  if  there  is  a  reasonable  doubt  as  to  the  copper 
deposits,  two  check  tests  shall  be  made  on  these  seven  wires  and  the  lot 
reported  in  accordance  with  the  majority  of  the  sets  of  tests. 

NOTE. — The  equipment  necessary  for  the  tests  herein  outlined  is  as 
follows : 

Filter  paper. 

Commercial  copper-sulphate  crystals. 

Chemically  pure  cupric-oxide  (CuO). 

Running  water. 

Warm  water  or  ice  as  per  needs. 

Carbona,  benzine  or  turpentine. 

Glass  jars  at  least  two  (2)  in.  inside  diameter  by  at  least  four  and  one-half 
(4>£)  in.  high. 

Glass  or  earthenware  jars  for  hardware  samples. 

Vessels  for  washing  samples. 

Tray  for  holding  jars  of  stock  solution. 

Jars,  bottles  and  porcelain  basket  for  stock  solution. 

Cotton  waste. 

Hydrometer  cylinder  three  (3)  in.  diameter  by  fifteen  (15)  in.  high. 

Thermometer  with  large  Fahrenheit  scale  correct  at  62°  and  68°. 

Hydrometer  correct  at  1.186  at  65°F. 


CHAPTER  XI 
LINE  MATERIAL 

Tie  Wires. — The  usual  function  of  the  wire  "pigtails,"  used 
to  attach  the  conductors  to  the  insulators,  is  to  prevent  falling 
and  creeping  rather  than  to  effect  dead-ending.  Even  assuming 
the  most  efficient  form  of  tie  attachment,  there  is  some  unknown 
relation  between  the  size  and  number  of  turns  of  the  tie  wire  and 
the  size  of  the  conductor  which  will  be  dead-ended  thereby. 
Further,  the  effectiveness  of  a  tie  wire  is  almost  entirely  depend- 
ent on  workmanship  and  the  attachment  must  be  made  without 
nicking  the  power  wire.  Stranded  cable,  particularly  in. large 
sizes,  can  be  tied  more  effectively  than  solid  wire,  owing  to  the 
grip  of  the  tie  between  the  strands. 

Tie  wires  should  either  be  made  of  the  same  material  as  the  con- 
ductors, or  be  coated  with  that  material  to  prevent  electro- 
lytic action.  The  wire  itself  must  be  "dead  soft"  or  free  from 
any  tendency  to  spring  loose  after  wrapping.  Many  kinds  of  ties 
have  been  devised,  although  they  are  all  variations  of  two  gen- 
eral types,  i.e.,  those  depending  upon  a  severe  constrictive  action 
in  one  or  two  turns,  and  those  in  which  friction  is  developed  over 
the  extended  surface  provided  by  a  number  of  turns.  The  latter 
type  is  generally  preferred,  especially  for  soft  copper  and  alu- 
minum, as  it  is  less  liable  to  injure  the  conductor. 

The  efficiency  of  the  tie  has  a  very  direct  bearing  on  the  con- 
ditions of  loading  which  can  logically  be  assumed  for  the  poles  or 
towers,  although  the  existence  of  this  relationship  seems  to  have 
been  generally  ignored.  Thus,  it  is  probable  that  in  very  many 
installations,  certain  assumptions  are  made  as  to  the  stresses  that 
may  be  transmitted  by  the  wires  to  the  supports,  without 
any  very  direct  information  either  as  to  the  type  of  the  tie  to  be 
used  or  its  ability  to  transmit  such  loads. 

For  example,  if  a  given  tie,  or  any  tie,  is  incapable  of  developing 
a  stress  of  5000  lb.,  a  No.  0000  copper  cable  cannot  exert  its 
maximum  tension  on  the  structure,  because  the  tie  would  fail 
before  transferring  such  a  load.  The  example  chosen  is  perhaps 

189 


190 


POLE  AND  TOWER  LINES 


unnecessarily  severe,  since  a  No.  0000  copper  cable  cannot  be 
successfully  dead-ended  by  any  tie  or  pin  insulator  with  which 


Tie  Wire 


Fig-Tall  or  Holding-Down  Tie. 
For  Aluminum  Conductor  use 
Strands  of  214  000  C.M.  Aluminu 
Cable  for  Holding-Down  lie. 
For  Copper  Conductor  use  No.10 
N.B.8.  Annealed  Telephone  (_ 

Wire  or  Annealed  Strands  of  No.  00 
Copper  Cable.  About  1%  Ft.  of 
Wire  Necessary. 


FIG.  111.— Ties. 


the  writer  is  familiar.     The  principle,  however,  applies  to  all 
cases  so  that,  except  at  corners,  the  poles  cannot  be  subjected  to 


LINE  MATERIAL 


191 


greater  wire  loads  than  the  ties  used  will  transmit.  The  writer 
does  not  infer  that  supports  in  general  have  been  made  too  strong, 
but  that  the  reasons  given  for  the  strengths  used  are  not  logical 
and  not  in  accordance  with  the  facts. 

Loops. — Loop  cables,  either  as  shown  in  Fig.  112  or  having 
Crosby  or  other  clips  instead  of  a  wrapped  splice,  have  some- 
times been  used  to  dead-end  or  securely  attach  the  conductors 
to  pin-type  insulators.  The  loop  is  placed  around  the  neck  of 
the  insulator  and  the  end  is  clamped  to  the  conductor  with  three- 
bolt  clamps  or  several  clips.  If  properly  made,  this  attachment 
has  a  much  greater  strength  than  the  average  insulator  or  pin, 
but  care  must  be  exercised  to  avoid  injuring  the  conductor. 
With  soft-drawn  cables  particularly,  any  attempt  to  secure 


FIG.  112.— Loop  cable. 

strength  by  overtightening  two  clips  will  probably  result  in  cut- 
ting the  strands  under  the  clips;  therefore,  three  or  more  clips 
should  be  used  in  order  to  reduce  the  grip  at  any  one  point. 

The  strength  of  loop  cables  is  primarily  a  matter  of  workman- 
ship, either  to  secure  an  efficient  splice  or  to  so  attach  the  loop 
to  the  power  wire  that  the  ultimate  strength  of  the  latter  may  be 
developed  without  injury  to  the  strands. 

A  few  tests,  incidental  to  others,  gave  the  following  results: 

Test  No.  1. — A  loop  cable  of  250,000  c.m.  soft  copper  directly  attached  to 
the  testing  machine  failed  in  the  loop  splice  at  7200  lb.,  after  developing  the 
entire  breaking  strength  of  the  cable. 

Test  No.  2. — A  similar  loop  cable  connected  to  the  conductor  with  Crosby 
clips  began  to  slip  at  2900  lb.  and  continued  tightening  of  the  clips  cut  the 
strands  of  the  cable. 

Test  No.  3. — A  loop  cable  of  ^-in.  steel  failed  at  9800  lb. 

Splices. — The  methods  of  splicing  conductors,  ground  wires 
and  telephone  wires  varies  considerably,  the  older  and  less  effi- 
cient forms  of  splice  being  still  quite  common  on  short-span  work. 
For  the  higher  voltage  longer  span  lines,  however,  splices  are  now 
generally  made  by  the  use  of  splicing  sleeves,  or  special  connectors. 


192 


POLE  AND  TOWER  LINES 


By  such  methods,  the  electrical  conductivity  may  be  maintained 
without  a  very  great  decrease  in  mechanical  strength.  Since 
hard  or  medium-hard  conductor  material  and  high-carbon  steel 
ground  wires  are  commonly  used  in  true  transmission-line  con- 
struction, it  is  not  advisable  to  do  any  soldering  or  sharp  bending 
at  splices.  The  former  anneals  the  conductor  and  very  consid- 
erably reduces  its  strength  while  the  latter  is  always  objection- 
able in  wires  subject  to  heavy  loading. 

Sleeve  splices  on  the  other  hand  may  be  given  any  desired  con- 
ductivity, combined  with  great  mechanical  strength.  Some  care 
is  necessary  in  the  selection  of  the  material,  length  of  the  sleeve, 
and  the  number  and  character  of  the  twists. 

Even  small  high-strength  solid  steel  wires  sometimes  used  for 
long-span  telephone  wires  can  now  be  successfully  spliced  by  steel 
sleeves. 


FIG.  113. 


FIG.  114. 


Pin  Insulators. — By  reference  to  the  section  on  insulator  pins, 
it  will  be  noted  that  few  if  any  pin  insulators  are  suitable  for 
turning  sharp  corners,  particularly  with  large  conductors.  Even 
providing  double  arms  will  not  give  factors  of  safety  commensu- 
rate with  those  presumably  required  on  the  supporting  construc- 
tion. This  weakness  of  the  pin  insulator  is  not,  accurately  speak- 
ing, chargeable  to  the  porcelain  but  to  the  pin.  To  use  stronger 
pins  would  require  larger  pin  holes  in  the  insulators  and,  there- 


LINE  MATERIAL  193 

fore,  new  designs  with  greater  neck  diameter.  However  much 
one  might  wish  that  designers  had  foreseen  the  present  tendency 
toward  rational  construction,  the  present  purchaser  of  small 
quantities  of  insulators  is  compelled  to  turn  to  a  different  type  of 
construction,  if  any  considerable  degree  of  strength  is  desired. 
The  stock,  or  standard  insulators  and  pins  shown  in  all  manu- 
facturers' catalogs  include  a  great  variety  of  designs  most  of 
which  could  be  dispensed  with  to  the  great  advantage  of 
both  the  manufacturer  and  purchaser.  By  handling  fewer 
designs  manufacturers  would  not  be  compelled  to  retain  so 
many  molds  and  so  much  stock,  while  purchasers  would  per- 
force have  their  choice  limited  to  a  smaller  number  of  well- 
selected  types.  Particular  reference  is  made  to  the  wood  top, 
porcelain  base,  wood  base,  or  all-wood-top  pins,  and  to  all  pins 
having  bolts  J,{Q  in.  or  J£  in.  in  diameter.  Mechanically,  the 
excuse  for  such  designs  in  their  application  to  severe  loading, 
defies  analysis. 

The  strength  of  an  insulator  or  of  a  pin  should  be  determined  by 
a  test  made  after  the  two  are  assembled  and  with  the  wire  and  its 
attachment  in  place.  Tests  on  pins  alone  may  involve  a  reduc- 
tion in  the  lever  arm  of  the  load,  and  in  any  case  give  no  indication 
of  the  result  of  pin  bending  on  the  insulator.  If  there  is  con- 
siderable bending,  which  is  exaggerated  on  wooden  arms,  the 
attachment  of  the  wire  may  slip  over  the  head  of  the  insulator. 
In  general  it  is  more  than  possible  that  the  values  obtained  by 
piece  tests  on  rigid  supports  could  not  be  even  approximately 
duplicated  in  actual  practice.  That  pin  insulators  have  an  en- 
tirely proper  function  may  not  be  denied.  Further,  they  are 
probably  the  most  economical  type  for  all  of  the  more  common 
voltages.  Although  used  up  to  80,000  volts,  with  the  expressed 
satisfaction  of  the  users,  the  writer  is  of  the  opinion,  taking  into 
consideration  the  mechanical  as  well  as  the  electrical  features, 
that  their  proper  field  of  usefulness  is  below  60,000  volts.  At 
heavy  corners,  however,  the  design  should  be  changed  to  the  disc 
type,  in  which  far  greater  strength  is  obtainable.  In  addition  it 
would  seem  entirely  logical  to  assert  that  if  pin-type  insulators  are 
used  on  tangents  and  the  insulators  in  question  have  only  a  fraction 
of  the  strength  of  the  wire,  then  the  supports  need  not  be  built  to  re- 
sist a  wire  tension,  which  the  insulators  can  never  transmit  to 
them. 

13 


194 


POLE  AND  TOWER  LINES 


There  may  be  reasons  for  desiring  a  certain  strength  in  the  sup- 
ports, but  certainly  broken-wire  loads  at  maximum  tension  on 
such  construction  cannot  be  one  of  them. 

Insulators  of  the  suspension  or  disc  type  are  more  satisfactory 
from  a  mechanical  viewpoint  than  pin  insulators,  since  they  trans- 
mit the  wire  tension  directly  to  the  crossarms  without  introducing 
the  torsion  due  to  the  height  of  a  pin  insulator.  When  used 
either  in  the  suspended  or  strain  position,  disc  insulators  require 
some  additional  height  of  pole  or  tower  and  an  additional  width 
of  crossarm  to  provide  clearance  for  the  jumper  or  the  suspended 


FIG.  115. — Suspension  insulators.  FIG.  116. — Through-pin  insulator. 

string  of  insulators.  When  used  in  the  strain  position  on  medium- 
voltage  lines,  it  is  now  thought  desirable  to  add  one  disc  to  the 
number  used  in  the  suspended  position.  For  the  high- voltage 
lines,  there  is  now  a  rather  general  objection  to  using  the  strain 
position  at  all  as  it  is  claimed  that  a  large  proportion  of  the  trouble 
from  lightning  has  occurred  at  strain  connections.  To  avoid  in- 
stalling insulators  in  the  strain  position  and  still  maintain,  at 
least  in  a  measure,  an  auxiliary  attachment,  the  arrangement 
shown  in  Fig.  162  has  been  used. 


LINE  MATERIAL 


195 


The  types  of  insulators  shown  in  Figs.  1 15, 1 16  and  117  are  much 
stronger  mechanically  than  single-pin  or  double-pin  insulators,  that 
in  Fig.  116  being  particularly  suited  for  turning  corners  with  heavy 
wires.  It  should  be  noted  that  the  through  pin  acts  as  a  simple 
beam  instead  of  as  a  cantilever,  and  transmits  its  load  to  double 
arms  without  torsion.  Further,  as  this  pin  can  be  made  a  rolled- 
steel  rod  about  1%  in.  in  diameter,  it  is  not  liable  to  bend  and 
crack  the  porcelain.  Unfortunately,  however,  the  electrical 
strength  of  these  insulators  does  not  render  them  advisable  for 
use  much  above  20,000  volts,  although  they  are  rated  as  30,000- 
volt  material.  Their  mechanical  strength  is  usually  about  12,000 


FIG.  117. — Heavy -strain  insulator. 

Ib.  A  corner,  particularly  an  important  one,  should  be  insulated 
approximately  up  to  crossing  requirements. 

Suspension  insulators,  in  general,  have  mechanical  strengths 
ranging  from  9000  Ib.  to  16,000  Ib. 

For  extremely  heavy  stresses,  strain  insulators  of  the  type  de- 
signed for  railway  service  and  shown  in  Fig.  117  may  be  used. 
These  insulators,  while  expensive,  can  be  equalled  in  strength  only 
by  some  paralleled  system  of  the  disc  type,  an  arrangement  both 
cumbersome  and  costly.  The  insulator  shown  has  a  mechanical 
strength  of  20,000  Ib.,  while  the  strength  of  a  larger  size  is  35,000 
Ib.  In  transmission-line  work  these  insulators  have  an  occasional 
use  in  very  long  span  construction. 

It  has  sometimes  been  specified  that  the  mechanical 
strength  of  guy  insulators  should  not  be  less  than  1,  1J£,  or  2 
times  that  of  the  guy  in  which  they  are  placed.  If  this  re- 
quirement were  strictly  enforced  it  would  penalize  the  use  of 
extra  heavy  guys  which  are  desirable  on  account  of  corrosion, 


196 


POLE  AND  TOWER  LINES 


since  it  would  be  difficult,  perhaps  impossible,  to  obtain  insulators 
of  the  requisite  strength.  Interlocking  insulators  of  the  "  goose- 
egg"  type  have  great  mechanical  strength  and  retain  a  measure  of 
the  guy  action  after  failure,  although  the  guy  would  be  slack. 
Such  insulators,  however,  do  not  have  the  high  electrical  factor 
of  safety  of  the  disc  type.  The  insulation  of  guys  on  poles  carry- 
ing high-voltage  wires  is  undesirable  from  a  structural  viewpoint, 
although  it  may  at  times  be  desirable  as  a  matter  of  policy  for 
the  protection  of  pedestrians,  particularly  in  the  case  of  wooden 
poles. 


FIG.  118. — Guy  insulators. 

Pins. — To  a  certain  extent,  the  day  of  the  wood  pin  is  passing, 
though  it  is  still  used  on  systems  operating  below  11,000  volts, 
and  apparently  gives  satisfactory  service.  Mechanically,  locust 
pins  are  inferior  to  metal  pins  and,  by  causing  the  removal  of  a 
large  section  of  the  timber,  materially  decrease  the  strength  of 
the  crossarms. 

It  is  generally  assumed  that  the  common  forms  of  straight-line 
insulator  pins  are  "  strong  enough,"  even  that  locust  pins  will 
"do  the  work."  It  is  true  that  almost  any  insulator  and  pin 
will  support  a  straight  unbroken  line,  but  it  is  not  therefore  true 
that  the  same  members  will  withstand  the  stresses  which  are  in 
the  wires  under  maximum  loading.  If  the  connection  of  the  wires 
to  the  insulators  will  not  transmit  more  than  a  fraction  of  the 
maximum  stress  in  the  wires,  then  the  pin  need  withstand  only 
the  transverse  load.  As,  however,  it  is  often  specified  that  the 
pins  must  dead-end  the  wires,  it  would  seem  necessary  first  to 
so  attach  the  wires  that  they  will  remain  attached  under  maximum 
tension  and  second  to  provide  a  pin  and  insulator  which  will 
remain  intact  under  this  stress. 


LINE  MATERIAL 


197 


Wood  pins  are  generally  of  yellow  or  black  locust  and  should  be 
straight  grained  and  free  from  knots  except  that  small  sound  knots, 
J£  in.  to  M  m-  diameter,  may  be  permitted  in  locations  where 
they  will  not  materially  impair  the  strength  or  durability  of  the 
pin.  They  should  be  free  from  wane  or  sap  wood,  and  from  checks 
or  worm  holes.  The  standard  thread  is  four  per  inch  and  the 
threads  should  be  cut  cleanly  and  uniformly  to  provide  a  tight 
fit  in  the  insulator.  Unless  well-seasoned  timber  is  used,  the 
pins  will  probably  vary  from  the  standard  dimensions  and  the 
protective  coatings  will  be  less  effective.  Such  coatings  are 
paint,  creosote,  linseed  oil,  and  perhaps  more  commonly  paraffin. 

The  holes  in  the  crossarm  should  be  so  bored  that  the  tapered 


FIG.  119. — Types  of  insulator  pins. 

shanks  of  the  pins  will  fit  tightly  therein  and  the  pin  be  per- 
pendicular. A  six-penny  nail  may  be  driven  into  the  shank  from 
the  middle  of  one  side  of  the  arm. 

Aside  from  the  possibility  of  rupturing  the  porcelain  in  case  the 
pin  bends,  it  has  been  found  by  test  that  certain  classes  of  pins 
deflect  as  a  whole  and  allow  the  top  of  the  insulator  to  be  subjected 
to  shear  and  tension.  If  such  pins  are  carried  by  wood  arms  the 
angular  movement  may  be  quite  large,  due  to  the  penetration  of 
the  pin  base  in  the  timber,  with  the  result  that  the  wire  fastening 


198 


POLE  AND  TOWER  LINES 


either  slips  over  the  head  of  the  insulator  or  shears  the  top  from 
the  insulator. 

It  is  probable  that  the  character  of  the  pin,  particularly  in  re- 
gard to  its  hold  on  the  insulator,  is  of  equal  importance  with  the 
strength  of  the  insulator  itself.  A  cemented  pin  is  somewhat  ob- 
jectionable because  it  cannot  be  readily  replaced  and  to  overcome 
this  difficulty  various  thimbles  and  separable-top  pins  have  been 
devised. 


i. 


FIG.  120. — Standard  locust  pin. 

The  writer  does  not  know  of  any  pin  having  a  wood  thimble 
in  which  the  strength  of  the  complete  pin  is  more  than  about  one- 
half  that  of  the  bolt.  This  is  due  to  the  fact  that  the  thimble  is 
too  thin  and  does  not  have  sufficient  bearing  on  the  base.  The 
weakest  part  of  this  type  of  pin  is  not  in  the  porcelain  insulator 
but  in  the  design  of  the  pin  itself. 


LINE  MATERIAL 


199 


Again  referring  to  the  cemented  pin  and  assuming  it  rigidly 
fastened  to  a  metal  crossarrn,  it  has  been  found  that  some  of  the 
ordinary  low-voltage  porcelain  insulators  and  metal  pins  cannot 
withstand  long-span  loading,  or  safely  support  the  transverse 
bending  caused  by  heavy  cables  on  angle  poles. 

On  high-voltage  lines  the  insulator  pins  should  be  of  metal. 
There  are,  however,  a  number  of  11,000  to  22,000- volt  lines 


c. . 


(a)          (6)  (c) 

FIG.  121. — Wooden  pins. 
F  C  E 

\%"  10"  2 

1%  13  3 

1U  17  3 


D 


equipped  with  wooden  pins.  The  low  mechanical  strength  of 
such  pins  and  the  possibility  of  their  disintegrating  or  burning 
has  raised  the  question  of  the  limiting  conditions  under  which 
wooden  pins  are  permissible. 

In  general,  pins  should  extend  well  into  the  insulator  to  reduce 
the  mechanical  stress  on  the  material  of  the  insulator.  On 
account  of  the  improbability  of  frequent  painting,  metal  pins 
should  be  galvanized,  or  otherwise  protected  against  corrosion. 


200 


POLE  AND  TOWER  LINES 


It  should  be  noted  that  in  case  of  a  broken  wire  some  of  the 
long  pins  now  in  use  would  develop  a  very  large  torsional  effect 
upon  the  crossarms. 

The  calculated  strengths  of  the  insulator  pins  shown  in  Figs. 
124,  125  and  126,  are  those  at  which  the  bolts  should  begin  to 
bend,  thereby  allowing  the  insulators  to  tilt.  In  making  the 
computations  it  was  assumed  that  there  was  a  complete,  level 
contact  between  the  pin  base  and  crossarm  and  that  the  bolt  was 


All  Wood  Top 


'Bolt 


FIG.  122. — Tension  applied  at  neck  of  insulator ;  average  of  three  tests.    500 
Ib. — pin  started  to  bend;  590  Ib. — pin  failed  by  splitting  the  wood  thimble. 

not  subjected  to  preliminary  bending  by  any  slack  in  the 
adjustment. 

If  wood  crossarms  are  used,  allowing  greater  preliminary  bend- 
ing due  to  the  bolt  compressing  the  fibers,  the  strength  and  ri- 
gidity of  the  construction  would  be  further  reduced.  On  the 
other  hand,  if  bolt  steel  having  a  yield  point  of  32,000  Ib.  were 
used  instead  of  the  steel  of  28,000  Ib.  assumed  above,  there  would 
be  an  increase  of  about  J£  in  the  tabulated  strengths. 

Wood-top  or  porcelain-base  pins  of  this  general  type  are  all 
relatively  weaker,  owing  to  the  fact  that  both  timber  and  porce- 


LINE  MATERIAL 


201 


lain   have   a   much   lower   crushing  resistance   than   cast-iron, 
malleable-iron,  or  cast-steel. 

The  following  conclusions  seem  to  be  justified:  (a)  That  most 
of  the  standard  designs  of  pins  now  in  use  are  undesirable  in  that 
the  metal  parts  are  weaker  than  the  porcelain;  (6)  that  ordinary 
insulator  pins  are  not  at  all  suitable  mechanically  for  corner  con- 
struction or  for  dead-ending;  (c)  that  much  stronger  pins  can  be 
designed  for  metal  arms  if  a  little  additional  thickness  is  allowed 
in  the  insulator  neck  and  a  larger  bolt  is  employed. 


FIG.  123. — Tension  applied  at  neck  of  insulator;  average  of  three  tests. 
770  Ib. — pin  started  to  bend;  920  Ib. — pin  failed  by  splitting  or  crushing 
the  wood  thimble. 

The  type  of  pin  shown  in  Fig.  127  was  designed  to  avoid  holes 
in  wood  crossarms.  It  would  appear,  however,  that  the  addi- 
tional cost  and  material  are  not  justified,  provided  comparison 
is  made  with  properly  designed  pins. 

Tests  have  shown  that  under  heavy  loading  the  critical  condi- 
tion is  often  not  the  strength  of  the  porcelain  or  of  the  metal  pin, 
but  the  ability  of  the  arm  to  resist  tilting.  If  the  arm,  as  a  whole, 
will  rotate  under  torsion,  or  if  the  base  of  the  pin  cuts  into  the 


202 


POLE  AND  TOWER  LINES 


timber,  or  twists  on  the  arm,  the  consequent  tilting  of  the  pin 
may  permit  the  wire  or  attachment  to  slip  over  the  head  of  the 
insulator  or  to  shear  the  top  from  the  insulator.  The  following 
short  series  of  tests  indicate  the  foregoing  tendencies: 


FIG.  124. — Cemented-type  all-metal  pin. 


L 

W 

B 

Elastic  limit  of 

bolt 

5K" 
6" 

7^r 

2H" 

2>r 

3" 

H" 

KG" 

7A4" 

p 
P 
P 

1  9^ 

v  ^finn  iv> 

1325  lb. 
540  lb. 
520  lb. 

5.25  X 

1  9^ 
V   ^fiOO  Ih 

"  6.0   X 

=  44  X  2600  lb.-  = 
/  .0 

Tension  at  neck  of  insulator,  transverse  to  arm. 

It  should  be  observed  that  the  strength  along  the  arm  in  the 
second  set  of  tests  is  considerably  in  excess  of  that  in  the  direction 
of  the  wires.  In  the  tests  in  question,  short  yellow-pine  arms 


LINE  MATERIAL 


203 


were  used  and  as  standard  length  arms  attached  to  a  pole  would 
have  less  rigidity  against  torsion,  the  effects  of  the  tilting  would 
have  been  aggravated  in  actual  practice. 


FIG.  125. — Cemented-type  all-metal  pin. 


L 

TF 

B 

Elastic  limit  of  bolt 

5K" 

3" 

X 

P 

=  H  X  8450  lb.  =  2300  lb. 

6H" 

3" 

H" 

P 

=  i^  X  8450  lb.  =  1950  lb. 

9H" 

4" 

K* 

P 

=  ^  X  8450  lb.  =  1780  lb. 

If  we  allow  a  factor  of  safety  of  2.0  in  the  pin  construction, 
and  assume  that  the  ultimate  resistance  to  failure  of  some  sort 
is  about  2000  lb.  for  single  arms  and  4000  lb.  for  double  arms, 


204 


POLE  AND  TOWER  LINES 


FIG.  126. — Separable-thimble  type  all-metal  pin. 


L 

w 

* 

Elastic  limit  of 

bolt 

8«" 

4" 

X" 

p 

=  |^    X  5480  Ib.  = 

1990  Ib. 

»H" 

4" 

K" 

p 

9  0 

=  H    X  8450  Ib.  = 

1780  Ib. 

13H 

5" 

H" 

p 

=  ~  X  8450  Ib.  = 

1690  Ib. 

lm 

5" 

H" 

p 

=  ^X84501b.= 

1560  Ib. 

*Test  No.  1. — 2380  Ib.,  insulator  uninjured,  excessive  pin  tilting,  bottom 
strap  bent. 

Test  No.  2. — 2400  Ib.,  insulator  failed,  excessive  pin  tilting,  bottom  strap 
bent.  Tension  at  neck  of  insulator,  along  axis  of  arm. 

Test  No.  3. — 2600  Ib.,  insulator  uninjured,  pin  tilted  cutting  into  wood  arm. 

Test  No.  4. — 3300  Ib.,  insulator  failed,  pin  tilted  cutting  into  wood  arm, 
bottom  strap  bent  and  split. 

Test  No.  5. — 4080  Ib.,  insulator  failed,  pin  tilted  cutting  into  wood  arm, 
bottom  strap  bent  and  split. 

the  maximum  wire  tensions  which  they  would  dead-end   are 
1000  Ib.  and  2000  Ib.,  respectively.     The  maximum  tension  in 
*  Tests  of  type  shown  in  Fig.  127. 


LINE  MATERIAL 


205 


conductors  larger  than  No.  1  gage,  however,  will  usually  be  in 
excess  of  2000  Ib. 

Crossarms. — The  standard  crossarm,  particularly  for  the  so- 
called  low-voltage  lines,  is  now,  and  will  undoubtedly  remain  for 
some  years,  a  wood  arm.  Even  in  view  of  the  great  increase  in 
the  price  of  timber,  the  wood  arm  is  the  cheapest  and  the  most 
easily  obtainable  throughout  the  country  as  a  whole.  Assuming 
that  the  price  of  wood  arms  will  continue  to  increase,  it  is  still 
probable  that,  for  some  years  to  come,  metal  or  other  materials 
will  not  seriously  compete  with  timber. 


^  ^'JO"" 

JL 


FIG.  127. 


FIG.  128. 


It  might  be  supposed  that  preservative  treatment,  which  will  un- 
doubtedly be  extensively  applied  to  poles,  would  prolong  the  use 
of  wood  arms.  To  some  extent  this  may  be  true,  but  while  the 
creosote  treatment  is  rapidly  growing  in  favor  for  poles,  the  same 
cannot  be  said  of  its  application  to  arms.  A  preservative  which 
would  make  arms  less  inflammable,  would  not  drip  on  passersby, 
and  would  not  injure  the  hands  or  clothing  of  workmen,  would  be 
more  desirable  for  crossarms  than  creosote. 

The  difficulty  of  standardization,  of  foretelling  accurately  the 
uses  to  which  an  arm  will  be  put,  makes  metal  arms  undesirable 


206  POLE  AND  TOWER  LINES 

for  small  growing  properties.  The  question  of  the  relative 
benefits  to  be  derived  from  the  insulating  qualities  of  a  wood  arm 
is  a  mooted  one.  Voltages,  at  least  those  below  13,000  volts,  can 
on  a  dry  day  be  successfully  insulated  by  the  wood  arm  alone. 

Under  such  conditions,  if  an  insulator  is  shattered  and  allows 
the  wire  to  fall  upon  a  dry  or  comparatively  dry  wood  arm,  no 
interruption  of  service  need  result  nor  is  there  any  injury  to  wire 
or  arm.  Some  unknown  additional  humidity,  or  degree  of  damp- 
ness of  the  arm,  will  cause  burning,  with  the  possible  falling  of 
the  arm  which  may  or  may  not  carry  other  wires  on  the  burned- 
off  portion.  The  advocates  of  the  metal  arm  contend  that 
it  is  economical  in  final  cost  and  that  the  insulators  should  be 


^4^12'^-12/^-12^--15V>^---15'-^-12':»-<-12'^7*-12V4^ 


"Ao   Through  Bolt  Hole^-19  -->t<— 19-  -^  /%'  Brace  Bolt  Hole — H  !  I*" 


8  Pin  Arm 


-  - 

11/16  Th 


4  Pin  Arm 

FIG.  129. — Standard  wood  arms. 

designed  to  do  all  the  insulating  necessary,  also  that  the  wires 
falling  on  a  metal  arm  shut  down  the  service  and  compel  proper 
maintenance. 

The  average  crossarm  would  not  withstand  dead  ending  under 
maximum  stress,  combined  with  the  torsional  effect  due  to  the 
lever  arm  of  a  long  pin,  without  allowing  a  distortion  which  would 
presumably  permit  the  wire  to  become  unfastened  from  the 
insulators. 

The  more  commonly  used  steel  arms  consist  of  single  angles 
with  the  same  general  dimensions  and  punching  as  standard  wood 
arms.  When  used  on  wood  poles,  crossarm  braces  are  necessary. 


LINE  MATERIAL 


207 


On  structural  steel  poles  to  which  crossarms  have  two  points  of 
connection,  braces  are  rarely  used,  as  it  is  simpler  and  nearly 


FIG.  130. — Substantial  crossarm  construction  for  eccentric  loading. 


FIG.  131. — Wish-bone  crossarms. 

as  economical  in  material  to  increase  the  section  of  the  crossarms 
themselves.  Except  with  painted  steel  poles,  and  sometimes  even 
in  that  case,  metal  arms  are  galvanized. 


208 


POLE  AND  TOWER  LINES 


In  addition  to  the  standard  angle  arms,  several  types  of  pat- 
ented arms  have  been  used  to  some  extent,  the  more  important 
being  the  so-called  " wishbone"  and  the  "bo-arrow"  arms. 
In  both  of  these,  adjoining  arms  are  brought  together  so  that  there 
may  be  two  points  of  attachment  to  counteract  rotation.  The 
upper  pole  bolt  must  not  be  placed  too  near  the  pole  top  as  the 
leverage  exerted  by  an  unbalanced  pull  on  an  arm  may  split  the 
pole  top.  The  use  of  special  arms  like  these  has  been  confined 
more  or  less  to  one  section  of  the  country,  and  it  is  perhaps  true 
that  familiarity  will  remove  the  sense  of  strangeness  with  which 
they  are  first  seen. 

Crossarm  Braces. — The  standard  brace  is  a  flat  \Y±  X  M"> 
26  in.  center  to  center  of  holes  and  28  in.  overall.  There  are 
also  various  modifications  of  the  standard,  such  as  changes  in 
length  and  reduction  of  thickness  to  %2  m-  or  Me  m- 

The  term  iron  is  still  in  common  use,  although  the  material  is 
usually  soft  steel.  In  fact  " wrought  iron"  or  soft  steel  is  fre- 


FIG.  132.— Angle  brace. 

quently  specified,  whereas  a  high-carbon  steel  would  be  more 
effective.  The  function  of  a  brace  is  to  support  and  prevent 
rotation  of  the  crossarm,  and  it  acts  in  either  tension  or  compres- 
sion. As  a  tension  member,  its  strength  is  excessive  and  its 
rigidity  against  buckling  as  a  column  is,  therefore,  the  critical 
condition.  Owing  to  the  shape  of  the  section,  the  rigidity  is  a 
function  of  the  thickness  and  the  strength  of  the  material,  and 
as  the  present  rather  ineffective  thicknesses  will  presumably  be 
retained,  some  added  stiffness  may  be  secured  by  the  use  of  the 
stronger  steels. 

The  angle  brace,  in  which  two  flat  braces  are  replaced  by 


LINE  MATERIAL 


209 


a  single  angle  about  1J£"  X  1M"  X  Me"  nas  a  much  greater 
strength,  but  almost  twice  the  weight  of  material  at  a  corres- 
ponding increase  in  cost. 

Braces  have  their  greatest  usefulness  when  the  crossarms  are 
unequally  loaded.  When  but  one  of  two  circuits  is  installed  or 
when  all  wires  are  placed  on  one  side  of  the  pole,  the  arms  will 
frequently  tilt,  particularly  on  lines  of  medium  voltages  carrying 
heavy  wires. 


FIG.  133. — Crossarm  with  two 
pole  bolts,  no  braces. 


FIG.  134.— Wooden 
braces. 


In  theory  at  least,  it  is  somewhat  undesirable  to  use  lag-screw 
connections  to  the  pole  or  to  the  arms  but,  in  practice,  the  timber 
about  the  screws  will  be  in  fab*  condition  when  replacement  of  the 
pole  or  a  change  of  arms  is  necessary  from  other  causes.  It  is 
probable,  however,  that  longer,  if  not  heavier,  lag  screws  should 
be  used  than  are  always  employed. 

Sometimes  crossarm  braces  have  been  omitted  and  two  through- 

14 


210  POLE  AND  TOWER  LINES 

bolts  used  to  connect  the  arm  to  the  pole.  This  is  not,  however, 
as  rigid  or  as  strong  as  the  more  standard  arrangement  of  one- 
pole  bolt  and  an  angle  brace.  When  a  short  crossarm  is  used  for 
the  top  wire  of  a  single-circuit  pole,  as  shown  in  Fig.  133,  the 
ground-wire  post  may  be  made  a  very  efficient  brace. 

Wood  braces  are  not  good  construction  as  it  is  difficult  to  ob- 
tain a  strong  permanent  connection  to  the  pole.  The  pole  shown 
in  Fig.  134  is  well  adapted  to  provide  adequate  longitudinal  pin 
separation,  something  which  is  not  readily  obtainable  in  double 
arming  with  large  insulators. 

Lag  Screws  or  Lag  Bolts. — The  fetter-drive  or  cone-point 
screws  generally  required  in  former  years  and  still  shown  as 
standard,  will  probably  be  replaced  by  the  gimlet-point  type  in 
future  work.  There  is  in  fact  no  possible  advantage  in  the  former 
and  the  continuance  of  a  double  standard  is  quite  objectionable 
from  a  manufacturing  standpoint. 


FIG.  135. — Cone-pointed  lag  bolt.     FIG.  136. — Gimlet-pointed  lag  bolt. 

In  theory,  lag  screws  are  supposed  to  be  screwed  into  place, 
either  into  a  small  bored  hole,  or  after  being  started  by  hammer- 
ing. This  should  be  done  to  enable  the  threads  to  pass  through 
the  timber  with  the  minimum  shearing  and  injury  to  the  fibers 
of  the  wood. 

Hardware  such  as  braces,  bolts,  lag  screws,  etc.,  should  be 
galvanized  by  the  hot-dip  process,  at  least  until  such  time  as 
other  methods  will  have  clearly  demonstrated  an  equal  excellence. 

It  has  sometimes  been  advocated  that  bolts,  etc.,  be  electrically 
galvanized  and  not  subjected  to  the  standard  test,  the  reason 
being  fairly  obvious. 

By  the  use  of  the  rolled  thread,  in  distinction  to  the  cut  thread, 
it  is  possible  to  use  the  hot-dip  process  without  recutting  the  bolt 
threads  and  thereby  removing  the  protective  coating. 

While  not  commonly  done,  nuts  may  be  made  with  extra  loose 
threads  and  after  galvanizing  retain  at  least  some  measure  of 
protection  on  the  threads.  It  is  probable,  however,  that  unpro- 
tected threads  on  the  nut  and  rolled  threads  on  the  bolt,  both 
being  hot-dipped,  are  superior  to  other  methods. 


LINE  MATERIAL  211 

Guys  and  Guying. — Guys  or  support  braces  are  of  three  types: 
timber  push  poles,  steel-cable  guys  and  rod  guys.  The  first  are 
unsightly  and  their  use  is  chiefly  justified  in  places  where  guys 
are  needed  in  two  directions  and  can  be  allowed  in  but  one. 
Provided  a  timber  brace  is  properly  set  and  well  connected  with 
the  line  pole,  it  is  capable  of  resisting  stresses  in  either  direction 
and  to  some  extent  may  act  as  longitudinal  reinforcement. 
When  used  in  such  double  service  the  setting  must  be  adapted  to 
resist  either  depression  or  uplift. 

Under  exceptional  conditions  rod  guys  may  be  used  as  they 
have  adjustable  connections,  form  a  rigid  anchorage  and  may 
be  made  with  an  excess  of  material  to  provide  for  corrosion.  In 
general,  however,  guys  are  of  stranded  galvanized-steel  cable 
and  when  properly  installed  are  a  component  part  of  the  support. 
The  writer  is  utterly  unable  to  agree  with  the  view,  sometimes 
expressed,  that  guys  are  a  makeshift  attachment  of  doubtful 
service.  In  fact,  it  is  his  firm  belief  that  only  the  good  service  of 
the  power  and  guy  wires  is  retaining  many  existing  lines  in  position. 

It  cannot  be  denied,  however,  that  there  are  many  guys  of  less 
than  no  usefulness  owing  to  extreme  slack,  inadequate  section,  or 
excessive  corrosion.  A  very  slack  guy  is  no  guy  at  all,  but  a 
small,  unsightly  load  on  the  structure. 

So-called  iron  wire  about  %  in.  in  diameter  is  not  an  efficient 
guy,  though  it  might  serve  for  very  light  lines  if  the  material 
were  in  reality  wrought  iron,  and  the  galvanizing  as  efficient  as 
would  be  desirable. 

Steel  cable  not  less  than  ^{Q  in.,  and  preferably  not  less  than 
%  in.,  with  a  heavy  coat  of  galvanizing  is  the  desirable  standard 
guy.  Greater  strength  may  be  obtained  by  the  use  of  larger 
diameters,  or  higher  grades  of  steel,  but  the  latter  are  stiffer  and 
more  difficult  to  handle  in  the  field  and  therefore  less  popular. 
It  is  possible  to  obtain  any  desired  grade  of  steel  cable  from  the 
standard-guy-strand  of  about  60,000  Ib.  per  square  inch  to 
extra-high-strength  steel  with  a  strength  of  180,000  Ib.  per  square 
inch.  In  general,  however,  the  standard  or  the  Siemens-Martin 
grades  should  be  used,  but  with  sufficient  diameter  to  provide 
ample  strength,  bearing  in  mind  the  much  more  rapid  corrosion 
of  galvanized  cable  than  of  galvanized  unwiped  structures. 

The  exact  location  of  guys  must  depend  on  local  conditions 
and  their  number  on  the  character  of  the  supports.  With  wood 


212  POLE  AND  TOWER  LINES 

poles  and  flexible  frames,  guys  should  be  used  more  plentifully 
than  with  semi-flexible  poles,  and  the  latter  in  turn  require 
more  guys  than  semi-rigid  towers,  while  the  true  rigid  towers 
require"  no  guys. 

In  general,  wood  poles  and  flexible  frames  should  be  side  guyed 
at  all  corners,  at  tops  of  steep  hills,  and  usually  wherever  a  very 
long  span  occurs.  They  should  be  head  guyed  on  steep  hillsides, 
long  spans,  hill  tops  and  at  intervals  on  long  tangents. 

There  is  no  valid  objection  to  the  intelligent  use  of  guys. 
Structures  so  designed  that  their  light  flimsy  nature  renders  them 
overliable  to  buckling  by  guys  should  not  be  used  in  a  transmis- 
sion line.  Further,  the  majority  of  the  existing  pole  lines  derive 
much  of  their  strength  from  guys.  //  a  guy  is  not  overtightened 
its  presence  must  inevitably  increase  the  strength  of  the  structure 
to  which  it  is  attached. 

In  guying  it  is  necessary  to  adapt  the  number,  size,  position 
and  tension  of  the  guys  to  the  service  required.  When  practi- 
cable, guys  should  not  be  anchored  too  close  to  the  structure 
they  support.  The  angle  of  inclination,  however,  is  not  fixed. 

The  insertion  of  a  turnbuckle  in  a  guy,  particularly  in  a  long 
guy,  permits  a  more  careful  adjustment  of  tension  than  is  practi- 
cable with  wire  clamps. 

Looping  a  guy  around  the  entire  tower  is  not  good  practice, 
except  in  unusu-al  cases  where  there  are  no  steel  edges  to  be  dis- 
torted and  where  the  structure  is  adequately  braced  both  verti- 
cally and  horizontally  at  the  point  of  guy  connection.  Guy  con- 
nections should  be  made  close  to  a  panel  point  of  the  vertical 
bracing  to  prevent  distortion  of  the  main  legs.  When  a  single 
guy  is  used  on  the  tension  side  of  a  tower  it  is  generally  desirable 
and  frequently  essential  to  attach  it  to  both  main  legs  on  that  side 
so  that  the  pull  will  be  exerted  squarely  on  the  tower,  and  not 
merely  on  one  corner.  Ordinarily,  a  guy  should  be  attached  as 
close  as  possible  to  the  conductors  whose  pull  it  carries  to  the 
anchorage.  Except  in  pole  lines  consisting  of  many  wires  where 
guys  attached  among  the  upper  crossarms  are  absolutely  neces- 
sary, the  guys  should  not  cross  over  conductors,  but  should  be 
connected  near  the  bottom  crossarm. 

With  steel  structures  guy  insulators  are  not  required  by  the 
standard  specifications,  their  use  being  optional,  but  if  used  they 
form  a  weak  point  in  the  guy. 


LINE  MATERIAL 


213 


Tt  =  total  tension,  or  pull  on 
pole  to  be  balanced  by  guy. 
Tg  =  total  tension  in  guy. 

0  =Lg  X  sin  a 

Assuming  that  L  —  Lg  =  3  ft., 
we  have: 


POLC 


FIG.  136a. 


TABLE  30. — TENSION  IN  GUY  DUE  TO  Tt  =  1000  LB. 

Lg  (ft.) 


(ft.) 

20 

25 

30 

35 

40 

45 

50 

5 

4,750 

5,710 

6,680 

7,680 

8,660 

9,670 

10,650 

10 

2,580 

3,020 

3,480 

3,950 

4,430 

4,920 

5,410 

15 

,920 

2,170 

2,460 

2,760 

3,060 

3,370 

3,690 

20 

,620 

,790 

,980 

2,190 

2,410 

2,620 

2,850 

25 

,470 

,580 

,720 

,870 

2,030 

2,200 

2,370 

30 

,380 

,460 

,550 

,670 

1,800 

1,920 

2,060 

35 

,320 

,380 

,450 

,530 

1,630 

1,740 

1,840 

40 

,290 

,320 

,380 

,440 

1,520 

1,610 

1,700 

45 

1,260 

,280 

,320 

,380 

1,440 

1,500 

1,590 

50 

1,240 

,260 

,290 

,320 

1,380 

1,430 

1,490 

While  guys  of  special  steel  are  sometimes  used  to  obtain  great 
strength  it  may  be  more  advisable  to  use  heavier 
guys  of  standard  material,  as  the  latter  is  more 
easily  handled  and  its  use  will  tend  to  encourage 
an  allowance  for  corrosion. 

Guy  Anchors. — Patented  guy  anchors  are  of 
many  kinds,  generally  variations  of  either  the 
old  patent  for  screw-piles,  or  of  the  unfolding 
type.  The  diameter  of  the  disc  or  blade  varies 
from  about  6  in.  in  the  smaller  sizes  to  12  in.  in 
the  largest.  The  holding  powers  claimed  for 
such  devices  should  be  used  with  the  reserva- 
tion of  a  factor  of  safety,  as  the  character  of  the 
soil,  either  in  general  or  at  the  time  of  test,  ex- 
ercises a  very  great  influence  on  all  foundation 
values. 

The  resistance  of  an  anchor  to  uplift  depends 
primarily  on  its  depth  and  bearing  area  and 
on  the  weight  and  cohesion  of  the  superim- 
posed soil.  The  depth  and  area  of  the  anchor  blade  are  lim- 


FIG.  137. 


214 


POLE  AND  TOWER  LINES 


ited  by  the  means  available  to  install  it,  but  the  weight  and 
cohesion  of  the  soil  will  vary  from  place  to  place  and  from  season 
to  season. 


Depending  on  the  Nature 
of  the  Rock 


FIG.  138. — Wooden  dead-man  guy  anchor. 

"Dead  men"  are  more  efficient  than  patent  anchors,  since  they 
are  always  larger  and  can  be  made  of  any  desired  size.  Large 
sound  logs,  about  10  in.  in  diameter  and  6  ft.  long  are  desirable 
for  ordinary  guying.  Logs  treated  with 
creosote  are  still  better,  while  the  best 
type  of  anchor  is  a  concrete-covered  steel 

1  In. Diameter  x&. 

is  in.  or  over  in  Length  \^  beam  or  reinforccd-concrcte  block.     In 

any  case  the  anchor  rod  or  rods  should 
be  galvanized  by  the  hot-dip  process 
(N.E.L.A.  Specifications)  and  preferably 
incased  in  concrete  or  in  a  concrete- 
filled  pipe  from  the  anchor  to  a  point 
about  1  ft.  above  the  ground  line. 

The   published  values  of  the  holding 
FIG.  139. — Rock  anchor. 

power  of  various  sized  anchors  are  ap- 
parently based  on  tests  in  clay  and  therefore  should  be  re- 
duced about  25  per  cent,  for  sandy  soil.  Further,  it  is  very 
difficult  to  arrive  at  any  acceptable  standard  values  for  the 


LINE  MATERIAL 


215 


holding  power  of  anchors,  as  would  be  evident  from  an  analysis 
of  the  values  heretofore  published.  For  instance, -if  the  holding 
power  of  a  6-in.  anchor  buried  vertically  .5  ft.  is  15,000  lb.,  the 
pressure  on  the  top  surface  of  the  6-in.  disc  whose  area  is  28.3 
sq.  in.  would  be  76,500  lb.  per  sq.  ft.,  or  38  tons  per  sq.  ft.  If 
the  holding  power  is  due  first  to  the  weight  of  the  cone  whose 
sides  have  an  inclination  of  45°  to  the  vertical  (and  such  in- 
clinations have  sometimes  been  limited  to  30°  in  foundation 


work),  and  second  to  the  cohesion  of  the  earth  on  the  periphery 
of  the  cone,  the  following  values  will  result: 
Angle  a  assumed  as  30°: 

Volume  of  superimposed  cone  of  earth  56  cu.  ft. 

Weight  of  cone  at  100  lb.  per  cubic  foot         =  5,600  lb. 

Cohesion,  or  shear,  at  150  lb.  per  square  foot  =  9,400  lb. 
Published  holding  power 


Angle  a  assumed  as  45°: 

Volume  of  superimposed  cone  of  earth 
Weight  of  cone  at  100  lb.  per  cubic  foot 
Cohesion  or  shear,  negligible 
Published  holding  power 


15,000  lb. 


=    151.5  cu.  ft. 
=  15,150  lb. 

= Olb. 

=  15,000  lb. 


The  pressure  of  38  tons  per  square  foot  is  greatly  in  excess  of 
that  permitted  by  any  foundation  specifications  and  the  inclu- 
sion of  cohesion  is  not  specifically  permitted  by  such  specifica- 
tions. Therefore,  it  would  seem  either  that  the  ordinary  require- 
ments for  foundations,  which  are  generally  assumed  as  having  a 


216  POLE  AND  TOWER  LINES 

factor  of  safety  of  5  or  6,  are  unnecessarily  conservative  for  anchor 
installations  in  fairly  good  ground,  or  that  the  generally  accepted 
holding  powers  of  anchors  are  excessive.  The  writer  believes 
the  former  to  be  the  case,  at  least  under  favorable  conditions, 
but  the  above  analysis  may  serve  to  explain  the  wide  discrepan- 
cies in  anchor  values.  It  must  be  admitted  that  the  efficiency 
of  any  given  type  or  size  of  anchor  will  depend  on  the  soil  in  which 
it  is  placed.  Therefore,  since  the  character  of  the  soil  at  various 
anchor  locations  is  not  usually  known  in  advance,  it  is,  perhaps, 
advisable  to  use  disc  or  unfolding  anchors  only  for  light  guys 
and  to  rely  on  the  installation  of  " dead-men"  to  resist  heavy 
stresses.  Far  greater  holding  power  can  be  obtained  by  the  use 
of  a  good  dead-man  than  can  possibly  be  provided  by  any  of  the 
patented  anchors  whose  area  is  necessarily  much  less  than  that  of 
any  dead-man.  Since  the  initial  stress  on  the  guy  anchor  will 
be  approximately  one-third  of  its  maximum  stress,  care  should 
be  taken  to  disturb  as  little  of  the  adjoining  earth  as  possible 
during  construction,  in  order  that  the  anchor  may  have  a  high 
initial  resistance  without  depending  on  the  additional  strength 
resulting  from  a  future  compacting  of  the  soil. 


CHAPTER  XII 
ERECTION  AND  COSTS 

Erection. — In  stringing  wires  it  is,  of  course,  of  importance  to 
cover  as  much  ground  daily  as  possible,  but  this  should  not  be 
done  at  the  expense  of  injury  to  such  an  important  and  expensive 
item  of  the  construction  as  the  wire.  Copper  wires,  whether 
solid  or  stranded,  cannot  be  dragged  without  injury  over  the 
ground  or  over  the  crossarms.  If  either  of  these  methods  is 
adopted,  it  will  result  in  nicks  in  the  solid  wire,  or  broken  strands 
in  the  cables.  Such  injuries  may  not  be  visible  and,  with  good 
fortune,  may  never  cause  failure,  but  anyone  who  has  seen  soft 
stranded  copper  wire  snarl  into  a  veritable  " rat's  nest"  when 
removed  through  snatch  blocks,  will  not  deny  that  injuries  may 
result  from  improper  stringing. 

It  is  also  necessary  to  be  constantly  on  the  lookout  to  avoid 
kinks,  twists,  or  broken  strands,  either  in  unreeling  or  in  stringing. 
When  broken  strands  or  injurious  kinks  do  occur,  a  new  section 
of  cable  should  be  spliced  into  the  line.  It  is  almost  impossible 
to  remove  the  cable  from  a  reel  without  forming  kinks  unless  the 
reel  rotates  about  its  axis.  The  reel  should,  therefore,  be  sup- 
ported on  a  horizontal  shaft  arranged  to  turn  freely,  but  not  too 
fast,  as  all  cable  has  a  tendency  to  kink  as  soon  as  a  little  slack 
occurs.  If  kinks  do  form  it  is,  of  course,  desirable  to  remove  them 
if  possible  to  do  so  without  injury  to  the  cable.  By  immediate 
attention  and  adherence  to  the  proper  methods  of  manipula- 
tion, it  is  possible  to  remove  a  kink  without  injury  to  the 
strands.  To  do  this  it  is  necessary,  as  shown  in  Figs.1  141,  1, 
2,  3  and  4,  to  straighten  the  wire  by  pushing  the  ends  apart 
without  altering  the  lay  of  the  strands.  If  this  is  not  done  cor- 
rectly, some  of  the  strands  will  be  stretched  and  the  spiral  of  the 
wires  will  be  distorted  at  the  point  of  bend,  and  thereafter  the 
cable  wih1  fail  by  the  parting  of  the  individual  strands  at  much 
less  than  the  ordinary  strength  of  the  cable. 

In  stringing  wires  they  should  be  pulled  out  through  snatch 

1  Illustrations  from  Yellow  Strand — Broderick  and  Bascom  Rope  Co. 

217 


218 


POLE  AND  TOWER  LINES 


blocks  with  wooden  sheave  and  frame  and  ball  or  special  bear- 
ings and  afterward  lifted  into  place  on  the  insulators  or  in  the 
clamps.  Any  simple  device  which  can  be  quickly  attached  to  a 


m 

•9 


\ 


FIG.  141. — Straightening  kinks. 

crossarm  to  hold  the  top  groove  of  the  snatch  block  at  the  eleva- 
tion of  the  clamp,  will  be  found  to  be  very  useful.  Periodic 
inspections  should  be  made  of  the  condition  of  the  snatch  blocks, 
to  prevent  injury  to  the  wire.  If  a  dynamometer  is  used  to  ad- 


ERECTION  AND  COSTS 


219 


FIG.  142. — Stringing  wire  with  derrick  car. 


FIG.  143. — Derrick  wagon  raising  pole. 


220  POLE  AND  TOWER  LINES 

just  the  sags  the  wires  can  be  transferred  from  the  blocks  to  their 
final  positions.  Dynamometer  stringing  is  particularly  desirable 
in  long-span  construction,  although  it  is  uncommon  in  the  more 
general  classes  of  short-span  work.  It  should  be  checked  oc- 
casionally by  measurement  of  the  resulting  sag.  It  is  advisable 
to  string  all  spans  to  balance  at  normal  temperature  and  no  wind 
or  ice,  even  though  this  results  in  some  unbalancing  under  load. 
Otherwise,  the  tensions  might  balance  at  maximum  load,  per- 
haps once  in  10  years,  and  be  unbalanced  the  rest  of  the  time, 
with  the  consequent  continuous  loading  of  their  supports. 

When  a  track  is  available  paralleling  the  pole  line,  erection 
with  a  derrick  car  will  give  the  greatest  possible  distances  per  day. 
In  the  absence  of  a  track  it  is  frequently  practicable  to  use  motor 
or  horse-driven  derrick  cars  for  the  erection  of  poles  or  for  string- 


FIG.  144. — Derrick  wagon.1 

ing  wires.  The  economy  of  such  erection  equipment  is  very 
great  under  favorable  conditions,  but  it  must  be  kept  moving  to 
attain  its  greatest  usefulness.  For  this  reason  no  direct  com- 
parison can  be  made  between  the  erection  costs  of  two  lines,  if 
one  is  long  and  accessible,  and  the  other  is  short  and  inaccessible. 

The  wagon  derrick,  shown  in  Figs.  143  and  144,  consists  of  a 
wide  stout  wagon  base  carrying  short,  double  shear  legs  from 
which  is  hung  a  wood  mast.  The  mast  is  hung  from  an  axle 
and  a  universal  joint,  thus  allowing  the  top  to  travel  in  an  arc 
limited  by  the  length  of  the  groove  which  restrains  the  base.  By 
this  arrangement  sufficient  overhang  is  obtained  to  reach  holes 
about  10  ft.  from  the  wagon.  The  mast  is  back  guyed  by  ropes 
attached  to  the  top  ring  and  snubbed  around  any  convenient 
object,  or  bar  driven  in  the  ground  beyond  the  wagon.  Since 

1  Designed  and  built  by  Wm.  A.  Ladue,  Supt.  Public  Service  Elec.  Co., 
Hoboken,  N.  J. 


ERECTION  AND  COSTS 


221 


the  height  of  the  shear  legs  is  not  great  and  as  the  mast  may  be 
rotated  in  both  directions,  the  rig  with  the  mast  in  the  horizontal 
and  axial  position  can  pass  under  bridges,  trolley  wires,  etc. 

When  but  one  of  two  ultimate  circuits  is  to  be  immediately 
installed,  as  shown  in  Fig.  145,  it  is  frequently  placed  on  one  side 
of  the  pole  and  on  the  highway  or  most  accessible  side.  This 
practice  is  not  commendable  as  it  tends  to  tilt  the  arms,  as  is 
faintly  visible  in  the  illustration,  and  because  erection  of  the 
second  circuit  is  made  much  more  difficult.  When  substantial 
angle  braces  are  used  such  eccentric  loading  is  probably  unob- 


FIG.  145. — Highway-side  loading. 

jectionable  under  ordinary  conditions,  but  it  has  been  observed 
that  all  side-arm  construction  suffers  more  in  severe  storms  than 
balanced  arms.  If,  however,  only  standard  flat  braces  are  used, 
some  arms  are  certain  to  become  tilted. 

Placing  the  first  wires  on  the  highway  side  of  a  pole  line  reduces 
slightly,  but  only  slightly,  the  original  cost  of  erection.  The 
second  circuit,  however,  will  have  to  be  erected  under  much  less 
favorable  conditions  than  the  first  would  have  been  if  placed  on 
the  inside.  It  is  better  construction,  when  both  circuits  are  to 
occupy  the  top  arm,  to  place  two  wires  on  the  inside  and  one  on 
the  outside.  This  will  necessitate  longer  shut-downs  on  the  first 


222 


POLE  AND  TOWER  LINES 


circuit  during  the  second  stringing,  but  it  will  reduce  the  ultimate 
cost  of  that  stringing  and  provide  a  stronger  original  line. 

Poles  should  be  set  vertically  and  in  line,  except  that  at  corners 
and  dead-ends  they  may  be  given  a  slight  rake,  though  this  is 
unusual  for  steel  poles.  In  building  foundations  or  setting  poles, 
particularly  in  long-span  lines,  it  is  necessary  to  exercise  some  care 
to  obtain  a  firm  unyielding  support  for  the  pole.  Since  providing 
a  layer  of  concrete  in  the  bottom  of  the  excavation  is  not  always 
practicable,  or  might  result  in  considerable  expense,  the  bottom 
of  the  excavation  should  be  compacted  by  tamping,  and  perhaps 
by  adding  and  tamping  a  6-in.  layer  of  broken  stone  or  gravel. 
The  back  filling  should  always  be  well  tamped  in  thin  layers,  and 
to  insure  this  it  is  frequently  required  that  one  shoveler  be  used 
to  three  tampers.  If  broken  stone  or  gravel  is  removed  from  the 


FIG.  146. — A  frame  erection  with  gin-pole. 

excavation,  or  readily  obtainable  nearby,  a  very  efficient  founda- 
tion may  be  obtained  by  back  filling  with  a  considerable  propor- 
tion of  rock,  being  careful  to  pack  earth  or  sand  in  the  spaces 
between  the  pieces.  Since  the  back  filling  will  settle  and  become 
more  compact  after  it  has  been  completed,  and  after  rains,  a  por- 
tion, at  least,  of  the  excess  excavation  should  be  piled  up  around 
the  base  of  the  pole.  Later  an  examination  s'hould  be  made  of 
the  foundations  and  back  filling  added  wherever  it  may  have 
settled  below  the  surface. 

On  wood-pole  lines  it  is  customary  to  set  adjacent  poles  with 
the  crossarm  gains  facing  opposite  ends  of  the  line.  Guys  should 
be  installed  before  any  wires  are  strung  and  should  be  inspected 


ERECTION  AND  COSTS 


223 


and  adjusted  if  necessary  after  the  stringing  is  completed,  other- 
wise, the  structures  may  receive  an  overloading,  while  without  the 
guys. 

If  some  wood  poles  have  unusually  large  tops,  the  regular 
crossarm  bolts  may  be  too  short.  In  such  cases  it  is  better  to 
obtain  a  few  long  bolts,  than  to  injure  and  weaken  the  top  of  the 
pole  by  cutting  it  down  to  the  shorter  bolts.  An  occasional 
heavy  pole,  or  one  with  a  large  top  and  regular  taper,  is  a  real 
asset  to  the  structural  strength  of  any  wood-pole  line,  therefore 
such  poles  should  not  be  weakened  by  excessive  top  cutting. 

Costs. — In  most  contract  work  it  is  fairly  accurate  to  assume 
that  in  general  the  work  will  approach  the  estimated  average 


FIG.  147. — Erection  with  house-derrick. 


and  that  under-estimates  of  some  portions  of  the  work  will  be 
balanced  by  exceptional  records  made  in  other  portions  where 
organization  and  familiarity  are  given  a  fair  test.  For  this  to  be 
true,  however,  it  is  necessary  that  there  be  a  fairly  large  volume  of 
work  in  a  few  locations.  Where  work  is  scattered  the  expense  is 
in  moving,  beginning,  and  stopping,  not  in  the  actual  work  itself. 
Further,  there  are  more  kinds  of  work  in  line  construction  than  in 
most  other  classes  of  contract  work,  since  each  pole  or  tower  loca- 
tion is  a  small  job  and  in  some  way  unlike  the  last.  It  is,  there- 
fore, impossible  to  attain  the  speed  of  piece-work  in  a  fixed  loca- 
tion, as  a  very  considerable  portion  of  an  employee's  time  is  spent 


224 


POLE  AND  TOWER  LINES 


in  "  thinking  "  about  the  next  step,  or  in  moving  to  a  new  position, 
or  in  getting  a  new  tool. 

In  a  comparatively  short  installation,  even  so  unconsidered  an 
item  as  a  specially  rainy  week  will  have  a  marked  effect  on  the 
unit  cost  per  structure.  Rain  or  snow  not  only  stops  or  delays 
progress — except  the  progress  of  the  "  straight-time"  pay-roll — 
on  the  day  in  question,  but  also  usually  delays  the  work  of  the 
following  day.  Holes  are  filled  with  water  or  snow,  equipment  or 
material  is  buried,  slides  have  occurred,  walking  and  teaming  is 
more  difficult,  and  in  general  it  is  a  poor  day's  rain  which  cannot 
count  as  two. 


FIG.  148. — Hauling  a  small  concrete  pole. 

Work  carried  on  between  spring  and  late  fall  should  cost  at  least 
20  per  cent,  less  for  labor  than  that  done  during  the  remainder 
of  the  year. 

Instances  are  rare  in  which  published  accounts  cover  the  matter 
of  accident  insurance.  An  owning  company  may  and  perhaps 
usually  does,  include  such  contingencies  in  overhead  expense,  but 
the  cost  is  nevertheless  directly  applicable  to  the  line  erection. 
A  contracting  concern,  on  the  other  hand,  usually  tabulates  the 
insurance  as  part  of  the  cost  estimate  and  as  it  is  almost  univer- 
sally paid  as  a  direct  percentage  on  the  actual  labor  pay-rolls,  it  is 
a  very  real  item.  The  amount  paid  for  insurance  varies  with 


ERECTION  AND  COSTS  225 

different  classes  of  labor  and  in  different  states.  Under  certain 
recent  legislative  enactments,  the  liability  of  employers  is  now 
only  partly  protected  by  premiums  as  high  as  15  per  cent.,  so  the 
casual  omission  of  such  items  of  expense  is  at  least  censurable. 

Again  in  comparing  the  costs  of  a  previously  established  method 
of  construction  for  any  company,  such  as  the  ordinary  wood-pole 
line,  with  for  example,  steel-pole  lines,  the  omission  of  general 
expense  items  in  the  former  is  quite  common.  For  instance, 
the  company  may  maintain  a  small  force  of  travelling  inspectors 
and  purchasing  agents  in  order  to  obtain  their  quota  of  wood 
poles,  and  this  expense  with  that  of  handling,  storage,  trimming, 
etc.,  is  properly  chargeable  to  the  cost  per  pole  delivered.  In 
general,  the  work  done  at  odd  times  on  regular  construction  mate- 


FIG.  149. — Raising  a  small  concrete  pole. 

rial  will  not  be  charged  thereto  as  certainly  as  the  unusual  charges 
to  new  types  of  construction. 

Proper  charges  for  plant  and  equipment,  particularly  for  small 
tools,  are  noticeable  by  their  absence  in  most  cost  estimates. 

It  has  been  stated  that  the  cost  of  steel  poles  or  towers  varies 
directly  as  the  square  of  the  height,  a  very  evident  error  since 
their  weight,  under  constant  conditions  of  design,  will  be  more 
nearly  in  direct  proportion  to  the  height,  and  the  greater  heights 
usually  indicate  a  smaller  number  of  structures  per  mile.  As  a 
matter  of  fact,  it  is  surprising  to  note  the  relatively  small  differ- 
ence in  estimated  cost  between  different  designs  of  equal  or  nearly 
equal  general  excellence.  The  conditions  of  manufacture,  accessi- 
bility of  the  site  and  character  of  the  ground  usually  influence 
the  ultimate  cost  much  more  than  is  often  realized.  In  addition, 


16 


226  POLE  AND  TOWER  LINES 

it  will  be  found  by  investigation  that  very  few  existing  lines  are 
directly  comparable  on  account  of  differences  in  design. 

Published  accounts  of  erection  costs  are  usually  misleading  and 
frequently  inaccurate.  Unless  the  local  conditions  are  similar 
and  the  methods  of  erection  equally  efficient  there  will  be  no 
equality  between  two  sets  of  costs.  Furthermore,  a  difference  in 
the  extent  of  the  work  and  in  the  organization  of  the  field  forces 
may  cause  a  relatively  great  difference  in  the  cost  of  two  lines  of 
exactly  similar  construction. 

In  making  a  comparative  cost  estimate  of  two  different  types 
of  construction,  as  for  example  wood  poles  and  steel  poles,  it  is 
necessary  either  to  make  two  complete  and  distinct  tabulations, 
or  to  use  care  to  include  all  credits  and  debits  due  to  the  differ- 
ences in  construction.  In  either  case  an  accurate  estimate 
should  include  allowances  for  maintenance,  renewals,  and  interest 
charges. 

In  making  estimates  it  should  be  remembered  that  the  use  of 
a  long-span  steel-pole  line  will  effect  a  saving  of  about  two-thirds 
of  the  cost  of  the  insulators,  pins,  ties,  pole  rights,  and  founda- 
tions, and  of  the  erection  of  the  insulators,  pins  and  ties.  Having 
fewer  insulators  than  the  shorter  span  wood  pole  line,  there  will 
be  less  probability  of  insulator  failure  and,  therefore,  less  inter- 
ruption to  'service.  In  addition,  some  credit  is  probably  due 
long-span  steel  construction  on  the  ground  that  the  maintenance 
expense  will  be  lower  and  that  a  high-grade  line  with  few  poles 
will  cause  less  criticism. 

If  shop-assembled  steel  poles  with  galvanized  butts  approxi- 
mately 24  in.  square  are  employed,  they  can  be  set  in  the  ground 
without  concreting  the  holes,  so  the  cost  of  the  foundations  per 
pole  should  not  greatly  exceed  that  for  wood  poles,  and  a  saving 
of  nearly  two-thirds  of  the  hole  digging  would  result. 

Steel  poles  are  distributed  and  erected  the  same  as  wood  poles, 
therefore  with  the  proper  field  equipment  a  mile  of  steel  poles 
should  be  set  at  least  as  quickly  as  a  mile  of  wood  poles. 

Established  costs  of  concrete-pole  lines  are  practically  non- 
existent, and  even  such  pole  costs  as  have  been  published  are 
rarely  applicable  to  ordinary  transmission  line  work.  The  more 
recent,  best  built,  and  most  important  concrete-pole  lines  have 
been  for  telephone  and  telegraph,  rather  than  power  service. 
Some  short  " back-yard"  poles  for  purely  distribution  service  have 
been  built  at  an  approximate  average  cost  of  $10  per  pole.  True 


ERECTION  AND  COSTS  227 

transmission  line  poles  of  adequate  height  and  strength,  unless 
made  in  quantities,  would  probably  cost  at  present,  about  the 
same  as  structural  steel  poles. 

As  previously  stated,  it  is  necessary  to  use  considerable  judg- 
ment in  making  cost  estimates  or  in  interpreting  them.  The 
following  costs  which  have  been  compiled  from  time  to  time,  as 
well  as  the  author's  estimates,  cannot  be  considered  universally 
applicable.  Indeed  they  are  only  reasonably  accurate  for  par- 
ticular cases. 

CALGARY  WOOD-POLE  LINE 
(Electrical  World,  Jan.,  1912) 

One  ground  wire,  24-in.  steel. 

Three  conductors,  No.  0  aluminum,  55,000  volts. 

Two  telephone  wires. 

Pin  insulators. 

Spans,  150  ft. 

Height,  40-ft.  poles. 

Average  cost  per  mile,  $2000. 

*   *   *  STEEL-POLE  LINE 

One  ground  wire,  Ke-in-  steel. 

Three  conductors,  No.  2  copper,  33,000  volts. 

Two  telephone  wires,  No.  10  copper-clad. 

Span,  400  ft. 

Height  of  poles,  43  ft. 

Poles $690 

Freight  and  cartage 25 

Foundations 65 

Guying 30 

Erection . .  55 


$865 

Wires $685 

Insulators,  pins  and  ties 145 

Erection..  105 


;  $935 

Clearing,  damages,  etc 25 

Right-of-way 195 

Supervision 100 

Miscellaneous.  .  110 


Total  cost. 


228  POLE  AND  TOWER  LINES 


*   *  WIDE-BASE  TOWER  LINE 

This  line  was  erected  under  adverse  weather  conditions,  and  in  extremely 
rough,  and  rather  inaccessible  country.     The  soil  was  hard  clay. 

One  ground  wire,  No.  00  copper. 
Six  conductors,  No.  00  copper. 
(One  circuit  installed). 
Standard  spans,  800  ft. 

Cost» 
per  tower 

Hauling $14 . 50 

Setting  footings 73.20 

Assembling  tower 24 . 80 

Raising 22.00 


$134.50 

Hauling — four-horse  teams,  driver  and  one  to  three  helpers. 
— a  portion  of  line  material  distributed  from  rail- 
road. 
Setting  — Foreman 

— four  diggers 

— six  to  eight  templet,  level  and  survey  men. 
Assembling — foreman    and    5    to   20  men,   depending   on 

weather. 
Raising — foreman,  four-horse   teams,  shear   legs,  etc.,  and 

eight  men  (average  four  towers  per  day). 
Cost  of  common  labor,  $2.25  a  day. 
1  Average  of  104  towers. 

*  STEEL-POLE  LINE  (NARROW-BASE  TOWERS) 

This  line  was  erected  in  1905,  under  circumstances  that  were 
not  at  all  favorable  to  a  low  cost.  The  work  was  done  in  winter 
weather;  the  foundations  were  expensive  both  in  design  and  in 
construction,  and  the  line  is  crooked  and  difficult  of  access 

Twenty-four  conductors,  250,000  circ.  mils,  soft-drawn 

stranded  copper,  11,000  volts. 
Eight  feeders,  500,000  circ.  mils,  soft-drawn  stranded 

copper,  650  volts. 
Wood  crossarms. 
Standard  spans,  150  ft. 
Pole  heights,  40,  45,  50,  and  55  ft. 

Weight  of  standard  40-ft.  pole 3000  Ib. 

Weight  of  standard  45-ft.  pole 3300  Ib. 

Weight  of  standard  50-ft.  pole 3800  Ib. 

Weight  of  standard  55-ft.  pole 4000  Ib. 

Weight  of  special  or  angle  poles .  . .   5000  Ib.  to  6000  Ib, 


ERECTION  AND  COSTS  229 

APPROXIMATE  AVERAGE  COSTS 

Poles  Cost  per  mile 

Steel  poles $3,550 

Wood  crossarms 200 

Erection 550 

*  Foundations 5,800 

Guying 50 

Painting  poles  and  arms 200 

Total $10,350 

ESTIMATED  COST.     ONE-CIRCUIT  LINE  WITH  STEEL  POLES 

One  ground  wire. 

Three  conductors,  No.  1  copper,  22,000  volts. 

Two  telephone  wires. 

Standard  spans,  450  ft 

Poles  per  mile,  12. 

Poles:  Cost  per  mile 

Material at  $50  per  pole     $600 

Freight 35 

Hauling at    $2 . 50  per  pole          30 

Foundations  (earth) 7  at    $2  =  $14 

Foundations   (braced) 3  at    $3=$9 

Foundations  (concrete) _2_at  $11  =  $22 

12  $45  45 

Erection at  $3  per  pole         35 

Guying 15 

Painting 20 

$780         $780 
Wires  and  Line  Material- 
One  ground  wire  %-in.  galvanized  steel  cable ....       $60 

Three  conductors,  No.  1  stranded  copper 670 

Two  telephone,  No.  6  BWG  Siemens-Martin  steel. . 80 

$810 

Ties,  guys,  splices,  etc 50 

33,000-volt  insulators  and  pins 
27  insulators  on  tangent  poles 
18  insulators  on  corner  poles 

45  insulators  and  pins at  $0. 75 each  =       $35 

45  telephone  insulators  and  pins .  .  at  $0 . 20  each  =       $10 

Hauling , 10 

Erecting 110 

$1025       $1025 

Clearing,  trimming,  etc 25 

Right-of-way at  $5  per  pole  60 

Supervision 100 

Contingencies  and  miscellaneous 25 

Total  per  mile  of  standard  line $2015 

Crossings  and  special  structures $ 


230  POLE  AND  TOWER  LINES 

The  following  comparative  estimates  of  the  costs  of  a  steel- 
pole  line  with  long-span  construction,  and  of  a  wood-pole  line 
with  short-span  construction,  both  for  the  same  location  in  access- 
ible rolling  country,  indicate  an  ultimate  saving  in  favor  of  the 
steel  line. 

ESTIMATED  COST.     ONE-CIRCUIT  LINE  WITH  WOOD  POLES 

One  ground  wire,  ^-in.  steel. 

Three  conductors,  No.  2  copper,  33,000  volts. 

Two  telephone  wires,  No.  10  copper-clad. 

Standard  spans,  120  ft. 

Poles  per  mile,  44. 

Pin-type  insulators. 

Metal  arms. 

Poles:  Cost  per  mile 

Poles  35  ft.  long,  7-in.  tops at  $5  each     $220 

Crossarms,  galvanized 167 

Telephone  brackets 5 

Pole  steps  and  hardware at  $0 . 75  per  pole         33 

Framing  and  trimming at  $0 . 50  per  pole         22 

Creosoting  butts at  $0 . 20  per  pole  9 

$456 

Hauling at  $1  per  Dole     $  44 

Digging  holes at  $1 .20  each  $  53 

Bog  shoes  or  braces 6 

Setting  poles at  $1 . 80  each       79 

Miscellaneous 4 

$142  $142 

Guying 30 

$672         $672 
Wires  and  Line  Material- 
One  ground  wire $  54 

Three  conductors 544 

Two  telephone 50 

$648 

Ties 5 

Soldering  materials 5 

33,000-volt  insulators 66 

Pins 49 

Telephone  insulators 5 

Ground-wire  connection 16 

Stringing  3  miles,  No.  2  copper 45 

Stringing  2  miles,  No.  10  copper-clad . .       20 

Stringing  1  mile,  %-in.  steel 18 

Miscellaneous 4 

$881  $881 


ERECTION  AND  COSTS  231 

Clearing,  trimming,  etc 10 

Miscellaneous  materials  and  tools 15 

Right-of-way at  $5  per  pole  220 

Supervision,  engineering  and  general  expense ....  100 

Contingencies  and  miscellaneous 25 

Total  per  mile  of  standard  line $1923 

Crossings  and  special  structures $ 

ESTIMATED  COST.     ONE-CIRCUIT  LINE  WITH  STEEL  POLES 

Wires  same  as  before,  except  that  a  Ke-in.  steel  ground 

wire  was  assumed. 
Standard  spans,  400  ft. 
Poles  per  mile,  13. 
Three-disc  suspension-type  insulators. 

Poles:  Cost  per  mile 

Material at  $53  per  pole     $689 

Hauling at      2 . 25  per  pole         29 

Digging  holes at  $1 .50  each  $19.50 

Concrete  at  corners 40 . 00 

Crushed  stone .6.00 

$65.50  $65 

Erection at  $2 .25  per  pole         29 

Guying 30 

Painting at  $1 .50  per  pole         20 

Miscellaneous -. .  8 

$870         $870 
Wires  and  Line  Material: 

One  ground  wire $75 

Three  conductors 544 

Two  telephone  wires 50 

$669 

Soldering  materials,  etc 5 

Insulators  and  clamps 137 

Telephone  insulators 5 

Stringing  3  miles,  No.  2  copper 54 

Stringing  2  miles,  No.  10  copper-clad.       24 

Stringing  1  mile,  Ke-in.  steel 20 

Miscellaneous 6 

$920  $920 

Clearing,  trimming,  etc 10 

Miscellaneous  materials  and  tools 20 

Right-of-way at  $7  per  pole  90 

Supervision,  engineering  and  general  expense. ..  100 

Contingencies  and  miscellaneous 25 

Total  per  mile  of  standard  line $2035 

Crossings  and  special  structures $ 


CHAPTER  XIII 
PROTECTION 

Ground  Wires. — In  the  light  of  our  present  knowledge, "ground" 
or  "sky"  wires  seem  to  be  desirable  on  lines  of  11,000  volts  or 
more,  but  are  not  necessary  on  lines  which  are  in  more  or  less  shel- 
tered locations.  If,  however,  the  ground  wire  is  of  less  durable 
material  than  the  conductors  under  it,  or  is  improperly  connected 


Ground  Wire 


FIG.  150. — Eccentric  location  of  ground  wire. 

to  the  supports,  it  becomes  a  menace  rather  than  a  safeguard.  A 
poorly  constructed  ground  wire  will  eventually  cause  interrup- 
tions in  the  service  of  the  power  wires  below  it. 

The  relative  merits  of  galvanized-steel,  galvanized-iron,  copper- 
covered  and  copper  wire  are  not  definitely  known  and  the  subject 
is  worthy  of  much  more  careful  consideration  than  it  has  thus  far 

232 


PROTECTION 


233 


received.  The  second  material  mentioned,  i.e.,  galvanized  iron, 
is  more  or  less  of  a  misnomer,  as  there  is  said  to  be  little  or  no 
real  iron  wire  used  for  transmission  purposes.  A  large  portion 
of  the  so-called  iron  wire  is  in  reality  soft  steel,  which  does  not 
have  the  ability  to  resist  corrosion  like  the  old-fashioned  wrought- 
iron  wire. 

In  the  process  of  galvanizing 
wire  cables  the  excess  coating  is 
wiped  off,  resulting  in  a  thinner 
coat  than  is  usually  obtained  on 
galvanized  shapes  which  are 
merely  allowed  to  drain.  This 
explains  in  some  measure  the 
increased  life  of  windmill  towers 
over  that  of  guy  cables.  It  need 
not  be  inferred  that  galvanized 
ground  wires  are  undesirable  in 
all  instances,  as  in  certain  locali- 
ties they  will  prove  economical. 
In  general,  however,  the  probable 
life  of  galvanized  wire  in  the  lo- 
cality under  consideration  should 
be  scrutinized  with  care  before 
such  material  is  placed  immedi- 
ately over  copper  conductors. 

The  choice  between  copper- 
covered  and  copper  cable,  is 
chiefly  a  matter  of  cost,  if  it  is 
assumed  that  the  relatively  thin 
shell  of  the  copper-covered  cable 
will  be  effective  in  preventing 
corrosion.  A  smaller  gage  cop- 
per covered  steel  wire  or  cable 

will  have  the  strength  necessary  to  permit  a  sag  equal  to  that  of 
the  larger  copper  cables.  If  a  copper-covered  ground  wire  is 
used,  it  should  have  a  heavy  coating  of  copper. 

A  number  of  lines  have  been  built  with  ground  wires  of  the 
same  section  and  material  as  the  conductors,  but  the  greater 
number  have  galvanized-steel  cable.  It  is  also  probable  that  the 
majority  of  ground  wires  are  of  a  smaller  gage  than  the  power 
wires.  Good  practice  seems  to  indicate,  however,  that  galvanized- 


FIG.  151. — Two-circuit  tower,  two 
ground  wires. 


234 


POLE  AND  TOWER  LINES 


steel  ground  wires  should  be  cables  %  in.  or  more  in  diameter  and 
that  copper-covered  stranded-steel  should  be  heavily  coated. 
Furthermore,  it  is  desirable  to  use  cables  having  few  strands  in 
order  to  obtain  a  relatively  thicker  coating  of  copper. 

The  ground  wire  connection  differs  from  the  power  connections 
in  that  it  should  be  treated  as  a  dead-end  connection  at  every  pole 
or  tower.  A  variation  in  sag  due  to  the  accidental,  or  intentional, 
slip  of  a  conductor  has  less  opportunity  to  cause  trouble  than  a 
similar  slip  in  the  ground  wire.  In  the  case  of  the  so-called  flex- 
ible towers,  by  which  is  meant  those  having  little  theoretical 

strength  in  the  direction  of  the 
line,  a  firmly  attached  ground 
wire  is  needed  to  serve  as  a 
partial  guy  to  help  minimize 
the  extent  of  tower  failure. 
The  ground  wire  attachments 
should,  therefore,  be  well  tight- 
ened, regardless  of  the  condition 
of  the  power-wire  attachments. 
On  some  supporting  struc- 
tures the  ground  wire  is  con- 
nected with  a  vertical  earth 
wire  leading  to  a  ground  plate 
beneath  the  support.  Such 
connections  should  be  arranged 
to  preserve,  as  much  as  possi- 
ble, the  original  strength  of  the 

ground  wire.  The  junction  is  necessarily  at  the  point  of  maxi- 
mum mechanical  stress  in  the  ground  wire;  therefore  soldered  or 
bent  connections  are  particularly  undesirable. 

The  proper  location  for  a  ground  wire  with  respect  to  the  con- 
ductors is  at  the  apex  of  a  60°  to  90°  angle  enclosing  the  latter 
wires.  In  recent  practice  the  ground  wire  is  usually  placed  a 
distance  above  the  upper  conductors  equal  to  about  one- 
half  the  horizontal  space  between  them.  On  some  high-voltage 
lines  with  two  circuits  in  vertical  spacing,  two  ground  wires  have 
been  used,  one  over  each  set  of  conductors.  This  method  un- 
doubtedly gives  some  increased  protection,  but  its  relative  effec- 
tiveness is  uncertain. 

On  the  other  hand  some  designs  for  one-circuit  poles  have 
placed  the  ground  wire  in  the  position  opposite  the  upper  insula- 


FIG.  152. — Crosby  clip. 


PROTECTION 


235 


tor  connection,  so  that  it  is  in  no  sense  over  two  of  the  power 
wires  (Fig.  150). 

It  seems,  therefore,  in  considering  the  protection  afforded  by 
overhead  ground  wires  and  in  judging  the  results  obtained  in 
actual  installations,  that  some  weight  should  be  given  to  the  rela- 
tive location  of  wires  on  the  lines  in  question. 

The  attachment  of  the  ground  wire  to  its  supporting  structure 


FIG.  153. — Ground  wire  clamping  cap. 

is  a  detail  of  great  importance.  If  a  long,  smooth,  well-rounded 
wire  seat  in  the  clamps  is  a  wise  provision  for  the  attachment  of 
the  power  wires,  it  is  equally  desirable  for  a  ground  wire  which  over- 
builds the  power  wires.  It  is  a  matter  of  common  knowledge  that 
a  short  rigid  metallic  connection  with  a  small  U-  or  hook-bolt 
biting  into  a  wire  has  a  tendency  to  cause  wire  failure.  Therefore, 
such  connections  should  not  be  used  in  the  very  worst  possible 
place  on  a  power  line. 


FIG.  154. — Ground  wire  clamping  cap. 

Further,  the  use  of  ridges  or  teeth  in  the  contact  surface  is 
decidedly  poor  practice  in  any  connection  to  copper  wire.  Such 
projections  merely  serve  as  cutting  edges  to  injure  the  softer 
copper  material  when  the  device  is  tightened.  The  indications 
are  that  better  results  are  obtained  with  copper  wires  by  the  use 
of  a  long  contact  surface  without  change  of  direction,  rather  than 
by  short  clamps  with  pronounced  waves.  Since  clamps  are  fre- 
quently galvanized,  care  should  be  taken  to  obtain  a  smooth  sur- 


236  POLE  AND  TOWER  LINES 

face  finish,  both  before  and  after  galvanizing,  on  the  portions 
which  will  come  into  contact  with  the  wire.  Sand  spots  or  edges 
on  the  black  material,  or  improper  draining  of  the  zinc  coating, 
may  cause  the  formation  of  sharp  projections  which  will  injure 
the  wire. 

The  ends  of  the  wire  grooves  should  be  bell-mouthed  with  a 
gradual  slope,  in  order  that  the  wire  may  not  be  bent  sharply,  or 
even  appreciably,  at  any  point.  In  other  words,  there  should  be  a 
reasonably  long  tangent  contact  surface  ending  in  curved  orifices, 
the  sides  of  which  should  confine  the  wire,  as  nearly  as  possible,  to 
the  exact  position  where  its  curve  of  sag  would  cause  it  to  lie  under 
any  conditions  of  loading. 

Neighboring  Lines. — Whether  or  not  the  location  of  a  proposed 
transmission  line  is  in  a  measure  fixed  by  existing  property  rights, 
as  may  be  the  case  with  lines  on  electric  railways,  it  is  necessary, 
or  at  least  very  advisable,  to  confer  with  the  owners  of  any  exist- 
ing and  adjacent  wire  lines  to  predetermine  what  measures,  if  any, 
may  be  required  to  prevent  interference  with  the  proper  operation 
of  such  foreign  lines. 

There  are  two  general  classes  of  neighboring  lines;  other  trans- 
mission lines,  and  the  so-called  "no-voltage"  lines  such  as  the 
telephone  and  telegraph. 

In  relation  to  both  of  these  classes  of  lines,  there  are  two  classes 
of  adjacency,  crossings  and  parallelism.  Crossings  may  vary 
from  single-span,  right-angle  crossings  to  several-span,  oblique  or 
"skew"  crossings.  The  term  parallelism  is  ordinarily  used  to 
indicate  two  lines  on  separate  structures,  though  it  also  applies 
to  over  building  whether  on  the  same  or  separate  structures. 

Interferences  may  also  be  divided  into  two  classes,  inductive 
and  contact.  Interferences  of  the  first  class  are  probably  con- 
fined to  cases  of  parallelism  while  the  latter  may  occur  with 
either  parallel  or  crossing  lines. 

The  theory  of  inductive  interference  is  not  yet  thoroughly 
understood,  so  generally  effective  measures  for  its  prevention 
have  yet  to  be  devised.  It  appears  that  induced  currents  may 
occur  with  rather  widely  separated  lines,  between  which  there 
can  be  no  physical  contact  either  direct  or  indirect.  The  matter 
of  induction,  therefore,  should  be  considered  as  a  separate  sub- 
ject. In  its  relation  to  the  physical  characteristics  of  a  trans- 
mission line,  induction  need,  therefore,  be  considered  only  as  a 
reason  for  the  inclusion  of  transpositions. 


PROTECTION  237 

Interferences  by  contact  may  be  of  many  kinds  and  may  occur 
wherever  two  lines  are  near  each  other.  Reasonable  security  at 
crossings  is  not  difficult  to  obtain,  but  the  matter  becomes" rather 
complicated  where  transmission  lines  and  no-voltage  lines  are 
closely  parallel. 

In  general,  it  will  be  found  advantageous  to  locate  the  proposed 
power  line  as  far  as  practicable  from  the  no-voltage  line,  prefer- 
ably on  a  different  route.  Otherwise,  the  two  lines  should  be 
separated  as  widely  as  possible,  and  ordinarily  occupy  opposite 
sides  of  the  highway  or  right-of-way.  In  some  instances  it  will 
be  necessary  to  move  all,  or  parts,  of  an  existing  line  in  order  to 
maintain  a  proper  separation.  Again  it  may  be  advisable  to 
consolidate  two  existing  lines  to  provide  space  for  the  power  line. 

In  view  of  the  fact  that  most  of  the  existing  lines  are  of  rela- 
tively remote  origin,  and  together  with  the  power  line  frequently 
subject  to  governmental  supervision,  it  should  be  unnecessary 
to  point  out  the  propriety  of  an  investigation  as  to  the  relative 
rights,  contracts  and  responsibilities  of  the  conflicting  lines.  Un- 
fortunately such  investigations  seem  to  have  been  the  exception 
rather  than  the  rule. 

In  an  attempt  to  reduce  the  possibilities  of  interference  by  con- 
tact, it  immediately  appears  that  physical  separation  is  the  most 
effective  method.  The  amount,  or  distance,  of  separation  is  not 
a  fixed  quantity,  nor  is  it  essentially  a  function  of  any  physical 
characteristic  of  either  line.  Each  installation  should  be  con- 
sidered as  a  special  case  since  the  local  topographical  conditions 
have  almost  as  much  bearing  on  the  effective  separation  as 'the 
pole  heights,  spans,  factors  of  safety  and  details  of  construction 
of  the  lines.  For  example,  it  is  evident  that  there  can  be  no 
physical  contact  between  the  supporting  structures  of  two  lines, 
if  they  are  separated  by  a  high  embankment. 

Again,  a  low  line  cannot  come  into  structural  contact  with  a 
tall  line  if  the  poles  of  the  short  line  are  set  opposite  or  near  the 
mid-span  of  the  other.  In  both  of  the  above  cases,  however, 
there  is  a  possibility  that  wind-blown  wires  of  either  class  may 
afford  contact  with  the  parallel  line.  The  relative  positions  of 
the  two  lines  with  regard  to  locations  on  side  hills  and  exposure 
to  storms  will  have  a  very  direct  bearing  on  the  possibility  of 
contact.  In  addition  to  the  foregoing,  consideration  must  be 
given  to  the  presence  of  inflammable  material  near  either  line, 


238 


POLE  AND  TOWER  LINES 


the  probability  of  wind-blown  branches,  etc.,  and  the  details  of 
construction  of  both  lines. 

An  investigation,  in  1914,  of  the  number  of  failures  at  cross- 
ings in  the  States  of  Idaho,  Oregon,  and  Washington  on  a  total 
of  1953  crossings,  for  periods  from  2  to  8  years,  showed  a  total 
of  six  failures,  only  one  of  which  resulted  in  the  damage  to  the 
company  crossed.1 


Voltages 

Crossing  years 

Failures 

Damage 

5,000-  7,000 

635 

0 

0 

11,000-15,000 

3,397 

4 

None 

22,000-44,000 

1,209 

0 

0 

55,000-66,000 

5,544 

2 

Phones  burned  ($25.00) 

10,785 

6 

Cradles. — A  cradle  or  guard  net  is  a  wire  basket — in  the  more 
elaborate  form,  a  wire  tunnel — formerly  used  to  separate  two 
power  systems,  or  to  protect  a  telephone,  telegraph,  or  similar 
system  from  an  electric  light  or  power  line.  A  cradle  to  be 
effective  must  practically  inclose  one  system  of  wires,  since  there 
is  no  justification  for  the  assumption  that  a  broken  wire  will  fall 
vertically  and  remain  within  the  confines  of  a  flat  net  of  restricted 
area. 

That  the  use  of  cradles  was  a  natural  development  in  the  prog- 
ress of  transmission  line  construction,  may  be  admitted;  it  is, 
however,  an  indisputable  fact  that  in  recent  years  they  have  fallen 
into  great  disfavor. 

Inasmuch  as  prevention  is  more  desirable  than  cure,  the  latest 
practice  in  this  country  is  to  so  install  the  power  line  that  the 
conductors  will  rarely  break  and  in  some  instances  to  further 
insure  against  a  falling  wire  by  the  use  of  an  auxiliary  attachment 
designed  to  hold  the  wire  in  case  of  the  failure  of  the  insulator 
or  of  the  wire  at  the  insulator. 

Clamping  Devices. — The  Joint  Report  specifications  for  cross- 
ings (Edition  of  1911)  state  explicitly  what  the  connection  of  the 
power  wires  to  the  supporting  structures  at  crossings  should  ac- 
complish, but  do  not  state  the  exa~ct  means  by  which  the  result 
should  be  attained. 

The  general  theory  of  the  clamping  device  is  to  require  an  effi- 


1  Idaho  Power  and  Light  Mens  Assoc. 


PROTECTION 


239 


•cient  insulator  and  a  positive  dead-ending  attachment  of  the  wire 
to  the  insulator  through  a  device  having  sufficient  mechanical 
strength  to  resist  the  tension  of  the  wire  in  case  the  insulator  fails ; 
that  the  device  should  have  sufficient  mass  to  resist  burning,  at 
least  to  some  extent;  and  that  the  points  of  attachment  to  the 
power  wires  be  at  a  sufficient  distance  from  the  insulator  to  mini- 
mize the  danger  from  arcs.  The  attachment  should  be  so  de- 


FIG.  155. 

signed  that  it  cannot  fall  free  of  the  pin  in  case  the  insulator  is 
shattered.  It  should  not  require  delicate  adjustment  and 
should  be  firmly  clamped  to  the  wire  without  injuring  it  in  any 
way. 

In  order  to  prevent  the  burning  of  wood  pins  and  crossarms  by 
arcs  from  defective  insulators,  or  fallen  wires,  by  causing  the 
circuit  breakers  to  act,  some  specifications  have  required  that 


ICT 


FIG.  156. 


FIG.  157. 


crossarms  should  be  of  metal,  or  be  provided  with  metallic  strips 
and  that  they  should  be  grounded.  In  other  instances,  metal 
grounding  arms  have  been  placed  below  the  wires  so  that  a  falling 
wire  would  come  into  contact  with  them,  as  in  the  H-frame  cross- 
ing, Fig.  45,  in  which  the  auxiliary  chain  attachments  have  not 
yet  been  clamped  to  the  power  wires.  On  the  other  hand,  the 
grounding  of  wood  crossarms  results  in  the  loss  of  the  insulating 


240 


POLE  AND  TOWER  LINES 


value  of  the  wood  arm.  In  dry  weather  a  wire  falling  on  a 
wood  arm  would  not  necessarily  burn  either  the  arm  or  the  wire. 

With  insulated  wire  there  is  a  possibility  of  injuring  the  wire 
in  removing  the  insulation  and  this  may  outweigh  the  effect  of  the 
insulation  in  preventing  a  tight  grip  on  the  power  wire  itself. 

It  is  not  clear  to  the  writer  how  a  device  at  the  support  can  in- 
sure against  accidents  from  breaks  out  in  the  span.  Moreover, 
it  would  appear  that  the  majority  of  failures  occur  at  the  insula- 
tors, so  that  the  greatest  practicable  benefit  would  be  obtained 
by  improving,  if  any  improvement  is  necessary,  the  construction 
at  the  insulators.  It  is  claimed  by  some  engineers  that  nothing  is 
required  except  a  first-class  insulator  and  a  tie  wire,  and  that  the 
cost  of  auxiliary  devices  would  better  be  expended  for  higher 
grade  insulators.  Further,  as  the  low-voltage  lines  are  not 
subject  to  much  electrical  trouble,  a  slight  increase  in  insu- 


FIG.  158. 


FIG.  159. 


lation  might  practically  insure  such  lines  against  failure  at  the 
insulators. 

In  some  cases  an  auxiliary  or  second  attachment  of  the  power 
wire  is  used,  but  unfortunately  this  is  not  always  as  effective  as 
it  appears.  For  instance,  if  such  an  arrangement  requires  dead- 
end connection  on  a  single-pin  insulator,  it  is  in  itself  undesirable 
and  mechanically  impracticable  for  heavy  stresses.  It  has  also 
been  proposed  that  the  wire  be  protected  from  arcs  by  the  use  of 
an  arcing  strip,  cap,  etc.,  but  these  are  not  effective  if  a  shattered 
insulator  allows  the  attachment  and  wire  to  fall.  Again,  it  is  not 
possible  by  the  use  of  any  device  at  the  support  to  prevent  a  wire 
broken  out  in  a  span  from  coming  into  contact  with  a  line  beneath 
it.  However,  as  the  majority  of  failures  occur  at  the  insulators, 
it  seems  wise  to  neglect  this  possibility  and  concentrate  attention 
on  other  features  of  the  connections  at  the  supports. 

In  the  writer's  opinion  too  little  value  has  been  attached  to  the 
ability  of  a  wire  to  hold,  over  a  doubled  span  length,  in  case  of 
pole  failure.  With  ordinary  short-span  construction  and  reason- 
ably low  voltages  there  is  little  reason  for  doubting  that  the  wires 


PROTECTION 


241 


will  be  less  liable  to  injury  if  the  crossarms  are  ungrounded. 
Therefore,  any  grounding  device  should  include  some  provision  to 
prevent  the  actual  separation  of  the  wire  into  two  spans,  either 
of  which  may  fall. 

The  clamping  device  or  dead-ending  attachment,  illustrated 
in  Fig.  160,  is  typical  of  the  erroneous  idea  that  protection  may  be 
afforded  foreign  interests  without  due  regard  to  the  protection  of 
the  power  line.  In  other  words,  the  benefits  from  the  method 


FIG.  160. 

used  to  eliminate  danger  from  falling  conductors  are  more  than 
offset  by  the  possibilities  of  accident  introduced  by  the  protective 
construction. 

As  shown,  the  span  adjoining  the  crossing  is  dead-ended  on  one 
pin  type  insulator  and  this  insulator  must  carry  a  heavy  me- 
chanical load,  not  merely  when  a  break  occurs,  but  at  all  times. 
Apart  from  the  fact  that  this  is  one  of  the  things  most  to  be 
avoided  in  line  material  installation  it  introduces  an  unbalanced 
load  on  the  support.  In  general,  it  compels  the  insulators,  pins 

16 


242 


POLE  AND  TOWER  LINES 


and  towers  to  withstand  a  continuous  loading  far  in  excess  of 
their  continuous  loading  under  ordinary  construction. 

The  crossing  span  is  supported  from  the  strain  insulator  con- 
nection and  the  power  cable  is  not  continuous  over  the  supports, 
the  clamps  having  to  serve  both  as  a  mechanical  and  an  electrical 
connection.  In  actual  construction  it  is  probable  that  the  power 
cable  would  be  passed  around  a  large  stout  thimble  and  possibly 
"served"  at  the  end  connection;  otherwise  the  detail  seems  an 
undesirable  one  for  copper  cable.  The  constant  mechanical 
stress  to  which  the  strain  insulators  are  subjected  is  undesirable 
and  at  high  voltages  might  be  very  objectionable,  though  it  is  true 
that  such  insulators  have  greater  mechanical  strength  than  those 
of  the  pin  type. 


Clamp 


Porcelain  Insulator, 
Metal  Pin 


FIG.   161. — Pin  insulators  with  caps  and  saddle. 


Further,  it  seems  at  least  possible  that  an  electrical  breakdown 
of  the  strain  insulators  might  cause  an  arc  which  would  melt  the 
jumper  cables  immediately  above  them  thus  rendering  the  aux- 
iliary connection  useless. 

The  purpose  of  the  grounding  arm  beneath  the  jumper  is  to 
ground  the  latter  in  case  a  conductor  breaks  outside  of  the  clamps, 
i.e.,  out  in  the  span.  There  does  not  appear  to  be  any  great 
assurance,  however,  that  the  ground  would  be  effective  before 
the  long  end  of  the  wire  falling  free  could  come  into  contact  with 
wires  beneath  it. 

The  use  of  the  rigid  clamping  cap  on  the  pin  insulators  is  not 
considered  the  best  practice,  the  general  tendency  now  being  to 
allow  the  power  cables  to  balance  themselves  about  a  smooth 
porcelain  surface,  and  to  have  any  auxiliary  connections  "ride" 
on  the  line. 


PROTECTION 


243 


If  there  is  any  basis  of  fact  in  the  recent  theory  that  lightning 
troubles  are  aggravated  by  bends  in  conductors,  this  construction 
would  be  open  to  such  criticism. 

Nests  of  insulators  supporting  a  cast  saddle  have  been  used  to 
some  extent  for  the  conductors  of  unusually  long  spans,  such  as 
river  crossings  and  in  some  instances  for  railroad  crossings 
(Fig.  161).  It  is  possible  in  this  manner  to  provide  sufficient  pin 
strength  to  withstand  heavy  stresses  and  at  the  same  time  obtain 
a  long  contact  surface  to  which  the  conductors  can  be  clamped. 
The  chief  objection  to  this  construction,  apart  from  the  cost,  is 
the  difficulty  of  removing  and  replacing  shattered  insulators. 
There  is  usually  a  considerable  weight  of  cable  carried  by  the 


;  j 

FIG.  162. — Two  strings  of  insulators,  suspended  position. 

saddle  and  also  a  vertical  component  of  the  wire  tension,  due  to 
the  fact  that  the  wires  often  descend  from  the  saddle  to  a  lower 
adjoining  pole.  The  clamping  attachment  of  the  saddle  as  well 
as  all  portions  of  the  wire  seat  should  be  smooth  and  well  rounded 
to  avoid  breakages  due  to  the  rigidity  of  the  saddle  and  the  vibra- 
tion of  the  wires. 

Another  and  perhaps  a  better  method  of  supporting  moderate- 
length  spans  is  to  use  two  strings  of  suspension  type  insulators 
hung  as  shown  in  Fig.  162.  Two  strings  of  insulators  in  the  strain 
position  have  also  been  used  and  have  generally  given  satisfactory 
service  except  at  the  higher  voltages. 

The  design  shown  in  Fig.  162  has  the  merit  of  a  lower  mechan- 
ical stress  in  the  insulators,  and  should  be  less  conducive  to 


244  POLE  AND  TOWER  LINES 

impact  in  case  of  wire  failure  while  retaining,  in  a  measure,  the 
auxiliary  or  double  connection.  This  method  of  attachment, 
however,  requires  considerable  separation  between  crossarms  to 
provide  clearance  for  wires  which  descend  sharply  on  one  side 
of  the  tower. 

Several  years'  experience  with  a  number  of  unusually  high 
river  crossing  towers  in  different  localities  has  led  the  writer  to 
believe  in  the  propriety  of  over-insulating  the  wire  attachments 
on  such  towers.  Considering  six  pairs  of  towers  ranging  in  height 
from  96  ft.  to  188  ft.  separated  from  10  to  100  miles  and  all  in  a 
region  subject  to  rather  severe  electrical  storms,  there  have  been 
but  two  cases  of  insulator  failure.  Both  insulators  were  of  the 
single-disc  type  and  were  hung  in  the  strain  position.  While  the 
insulators  were  badly  burnt  and  shattered,  in  neither  case  did 
they  break  apart  or  allow  the  wire  to  fall.  Two  crossings  have 
pin  insulators  and  the  others  have  disc  insulators  in  the  strain 
position.  The  failures  occurred  on  one  of  the  two  sets  which  did 
not  have  an  overhead  ground  wire.  One  failure  resulted  directly 
from  lightning  and  the  other  apparently  from  a  heavy  surge  fol- 
lowing a  head  puncture,  by  lightning,  of  a  pin  insulator  on  an- 
other structure.  All  of  the  towers  are  grounded.  No  other 
trouble  of  any  nature  has  been  reported  on  any  other  wires, 
although  the  various  structures  carry  from  six  to  twenty-four 
13,000-volt  wires  each. 

DISCUSSION  OF  JOINT  REPORT 

SPECIFICATIONS   FOR   CROSSINGS 
(Edition  of  1911-1914) 

The  writer  was  a  member  of  two  committees  signing  the  Joint  Report 
and  perhaps  for  the  very  reason  that  he  has  been  compelled  to  occupy 
a  double  standpoint  some  explanatory  discussion  of  various  disputed 
requirements  may  not  be  out  of  place.  The  fact  is  frequently  forgotten 
that  this  specification  is  a  pioneer  and  in  its  very  nature  a  compromise. 
There  was  previously  no  standard  even  approximately  acceptable  to 
conflicting  interests,  whereas  with  the  Joint  Report  as  a  basis,  an  electric 
service  company  and  any  company  whose  lines  are  crossed  are  able  to 
agree  promptly  on  mutually  satisfactory  construction  without  the  old 
interminable  delay. 

Objections  have  been  made  that  the  specifications,  when  literally 
enforced,  are  oppressive  in  certain  special  cases.  This  is  not  a  reasonable 
objection  since  it  may  be  said  with  equal  truth  of  any  specification. 


PROTECTION  245 

To  consider  the  specifications  by  clauses  we  have: 

1.  SCOPE. — The  limitation  of  5000  volts  in  relation  to  telephone  lines, 
while  no  doubt  desirable  from  a  telepone  standpoint,  may  be  a  hardship 
to  power  lines  located  in  streets.  If  it  is  a  fact  that  protective  apparatus 
is  not  effective  above  2300  volts,  there  seems  little  logic  in  limiting  the 
power  line  to  5000  volts,  if  the  probability  of  failure  on  a  well-constructed 
13,000- volt  is  not  measurably  greater  than  that  of  the  ordinary  2300- 
volt  line.  The  limitation  is  an  old  standard  which  conceivably  should 
not  now  apply  with  equal  force  under  improved  methods  of  construction. 

The  words  "  constructed  over'*  are  perhaps  unfortunate  since  they  in- 
troduce the  inference  that  the  clause  applies  to  joint  poles  and  parallel 
lines.  In  reality  the  specification  is  as  stated  a  crossing  specification,  a 
crossing  being  a  single  span  or  perhaps  two  or  three  spans. 

8.  CLEARANCE. — Side  clearances  of  (12  ft.  0  in.)  are  not  properly  en- 
forceable in  case  the  track  occupies  a  city  street  within  that  distance 
of  the  curb,  provided  always  that  reasonable  clearance  may  be  obtained, 
or  that  trees,  etc.,  already  occupy  the  curb  line. 

10.  While  the  8  ft.  0  in.  clearance  above  other  wires  is  reduced  by 
paragraphs  16  and  33,  to  6  ft.  0  in.  under  certain  conditions,  it  would 
be  an  improvement  to  specifically  permit  6  ft.  0  in.  clearance  for  all  con- 
struction in  which  the  wires  are  securely  fastened  and  when  the  poles 
are  not  subjected  to  much  bending.  Further,  such  clearances  should 
be  extended  to  cover  the  voltages  above  those  embraced  in  the  various 
clearances  given  in  the  specifications. 

12.  Dead-ending  through  disc  insulators  in  the  strain  position  should 
not  be  mandatory  as  more  recent  experience  tends  to  prove  this  method 
undesirable  in  some  instances.  Provided  other  requirements  are  com- 
plied with,  this  clause  should  not  be  blindly  enforced. 

19.  GUYS. — While  inferentially  guys  are  permitted  on  any  type  of 
structure,  they  should  be  specifically  permitted  with  proper  regulation. 

23.  GROUNDING. — As  indicated  heretofore,  the  writer,  personally, 
is  not  in  favor  of  a  general  enforcement  of  the  grounded  arm  and  in  view 
of  the  general  lack  of  agreement  on  the  subject  since  developed  by 
the  specification,  it  would  seem  preferable  to  make  the  requirement 
voluntary. 

34.  LOADS. — The  sliding  scale  of  broken  wires,  while  apparently  a 
reasonable  compromise  between  none  broken  and  all  broken,  should 
specifically  include  provision  for  designing  with  "pullback"  from  the 
unbroken  wires.  This  is  what  actually  occurs  in  fact  on  heavy  lines 
and  unless  this  interpretation  is  made,  such  lines  are  unduly  punished. 
Further,  it  is  illogical  to  design  all  multiple-pin  arms  for  large  broken 
cables  without  pullback.  Pullback  is  not  mentioned  directly  in  the 
specification,  though  it  may  be  inferred. 

37.  FACTORS  OF  SAFETY. — The  pin  and  insulator  are  a  structural  unit 
after  erection  and  the  porcelain  does  not  really  need  a  larger  factor 


246  POLE  AND  TOWER  LINES 

than  the  pin,  nor  is  its  individual  factor  readily  determinable.  For  heavy 
cables  the  factors  given  for  pins  and  insulators  are  prohibitory  if  liter- 
ally enforced. 

The  factor  of  3.0  for  structural  steel  is  not  exactly  correct  as  a  single 
statement  and  should  be  omitted  since  the  matter  is  covered  by  the 
allowable  unit  stresses  given  in  paragraph  69. 

Concrejbe  poles  if  properly  made  are  unnecessarily  penalized  if  a 
factor  of  4.0  is  used. 

The  factor  of  2.0  for  foundations  should  not  be  blindly  enforced  as 
the  conditions  of  loading  presuppose  at  least  a  semi-frozen  soil  and 
the  general  methods  of  computing  foundations  err  on  the  side  of  safety. 

45.  CONDUCTORS.— There  seems  to  be  little  justice  in  using  this 
paragraph  to  require  an  existing  solid  wire  on  ordinary  construction  to  be 
replaced  by  a  stranded  cable.  The  clause  is  founded  on  the  greater 
strength  of  large  stranded  cables  for  long-span  construction  and  on 
the  greater  chance  of  injury  to  solid  wires  in  erection.  Therefore, 
short  spans  and  careful  erection,  particularly  with  insulated  wire, 
would  be  unjustly  treated  by  a  literal  and  general  enforcement. 

48.  INSULATORS. — The  strength  of  the  guy  insulator  should  be  twice 
that  of  the  guy  stress,  not  of  the  guy  strength.  This  is  evidently  a 
confusion  of  intent  since  the  latter  would  penalize  extra  strong  guys 
used  for  protection  against  corrosion. 

Again,  the  interlocking  feature  is  not  generally  applicable,  indeed 
impracticable  for  high  voltages  and  the  requirement  should  not  be 
mandatory. 

54.  CONCRETE. — At  the  time  the  specifications  were  written,  the  then 
forthcoming  Joint  Committee  Report  on  Concrete  and  Reinforced 
Concrete  seemed  the  most  general  authority  on  the  subject.  There  is 
now  grave  question,  however,  that  it  can  be  applied  to  crossings,  since  it 
covers  a  different  field  of  work,  i.e.,  bridges  and  buildings,  and  is  only 
fairly  enforceable  in  certain  individual  requirements,  though  education- 
ally of  benefit. 

59.  STRUCTURAL  STEEL. — Reference   to   the  preceding   sections  on 
structural  design  will  show  that  the  use  of  large  ratios  of  l/r  are  not 
conducive  to  the  best  types  of  construction,  nor  safely  permissible  for 
general  use.     Some  reduction  in  the  figures  given,   such  as  to  150 
and  200,  would  be  no  great  hardship  and  of  some  real  benefit  to  the 
general  excellence  of  future  work. 

60.  Similar  reasoning  indicates  an  increase  in  minimum  thickness  to 
%  e  in.  for  galvanized  material. 

64.  FOUNDATIONS. — Although  accurate  data  on  foundation  design  are 
wanting,  the  strict  application  of  the  clause  works  a  hardship  by  in- 
ferentially  omitting  the  unknown  value  of  earth  shear,  arch  action,  etc. 

73.  TIMBER. — Paragraphs  37  and  73  seem  unnecessarily  conservative, 
particularly  for  selected  timber  treated  with  preservative,  or  whose 


PROTECTION  247 

deterioration  will  be  closely  watched.  It  may  be  granted  that  a  fair 
and  accurate  requirement  is  almost  impossible,  but  some  increase  of  unit 
stress  is  certainly  reasonable  for  lines  in  city  streets. 

In  conclusion  and  in  spite  of  the  foregoing  interpretations  many  of 
which  have  been  developed  only  by  actual  use,  the  writer  again  repeats 
his  firm  belief  that  the  specifications,  if  used  with  an  honest  desire  to 
correlate  divergent  interests,  are  a  very  efficient  work  and  a  great  advance 
on  all  previous  measures. 


CHAPTER  XIV 

JOINT  REPORT  SPECIFICATIONS  FOR  OVERHEAD 

CROSSINGS  OF  ELECTRIC  LIGHT  AND 

POWER  LINES 

(Edition  of  1911-1914) 
GENERAL  REQUIREMENTS 

1.  Scope. — This  specification  shall  apply  to  overhead  electric  light 
and  power  line  crossings  (except  trolley  contact  wires),  over  railroad 
right-of-way,  tracks,  or  lines  of  wires;  and,  further,  these  specifications 
shall  apply  to  overhead  electric  light  and  power  wires  of  over  5000 
volts  constant  potential,  crossing  telephone,  telegraph  or  other  similar 
lines.  It  is  not  intended  that  these  specifications  shall  apply  to  crossings 
over  individual  twisted  pair  drop  wires,  or  other  circuits  of  minor 
importance  where  equally  effective  protection  may  be  secured  more 
economically  by  other  methods  of  construction. 

2. Location. — The  poles,  or  towers,  supporting  the  crossing  span 
preferably  shall  be  outside  the  railroad  company's  right-of-way. 

3.  Unusually  long  crossing  spans  shall  be  avoided  wherever  practicable 
and  the  difference  in  length  of  the  crossing  and  adjoining  spans  generally 
shall  be  not  more  than  50  per  cent,  of  the  length  of  the  crossing  span. 

4.  The  poles,  or  towers,  shall  be  located  as  far  as  practicable  from 
inflammable  material  or  structures. 

5.  The  poles,  or  towers,  supporting  the  crossing  span,  and  the  adjoin- 
ing span  on  each  side,  preferably  shall  be  in  a  straight  line. 

6.  The  wires,  or  cables,  shall  cross  over  telegraph,   telephone  and 
similar  wires  wherever  practicable. 

7.  Cradles,  or  overhead  bridges,  shall  not  be  used  beneath  the  cross- 
ing wires  or  cables ;  but  in  cases  where  the  crossing  wires  or  cables  cross 
beneath  the  railroad  wires,  telephone,  telegraph,  or  other  similar  wires, 
a  protection  of  adequate  strength  and  proper  design  between  the  two 
sets  of  crossing  wires  or  cables  may  be  required. 

8.  Unless  physical  conditions  or   municipal  requirements  prevent, 
the  side  clearance  shall  be  not  less  than  twelve  (12)  ft.  from  the  nearest 
track  rail,  except  that  at  sidings  a  clearance  of  not  less  than  seven  (7) 
ft.  may  be  allowed.    At  loading  sidings  sufficient  space  shall  be  left 
for  a  driveway. 

9.  The  clear  headroom  shall  be  not  less  than  thirty  (30)  ft.  above 

248 


OVERHEAD  CROSSINGS  249 

the  top  of  rail  under  the  most  unfavorable  condition  of  temperature 
and  loading.  For  constant  potential,  direct-current  circuits,  not  exceed- 
ing 750  volts,  when  paralleled  by  trolley  contact  wires,  the  clear  head- 
room need  not  exceed  twenty-five  (25)  ft. 

10.  The  clearance  of  alternating-current  circuits  above  any  existing 
wires,  under  the  most  unfavorable  condition  of  temperature  and  load- 
ing, shall  be  not  less  than  eight  (8)  ft.  wherever  possible.     For  con- 
stant potential,  direct-current  circuits,  not  exceeding  750  volts,  the 
minimum  clearance  above  telegraph,  telephone,  and  similar  wires  may 
be  two  (2)  ft.  with  insulated  wires  and  four  (4)  ft.  with  bare  wires. 

11.  The  separation  of  conductors  carrying  alternating  current  sup- 
ported by  pin  insulators,  for  spans  not  exceeding  150  ft.,  shall  be  not 
less  than: 

Line  voltage  Separation 

Not  exceeding  7,000  volts ; .  12  in. 

Exceeding    7,000,  but  not  exceeding  14,000 •  20  in. 

Exceeding  14,000,  but  not  exceeding  27,000 30  in. 

Exceeding  27,000,  but  not  exceeding  35,000 36  in. 

Exceeding  35,000,  but  not  exceeding  47,000 45  in. 

Exceeding  47,000,  but  not  exceeding  70,000 60  in. 

For  spans  exceeding  150  ft.  the  pin  spacing  should  be  increased, 
depending  upon  the  length  of  the  span  and  the  sag  of  the  conductors.1 

With  constant  potential,  direct-current  circuits  "not  exceeding  750 
volts,  the  minimum  spacing  shall  be  ten  (10)  in. 

12.  When  supported  by  insulators  of  the  disc  or  suspension  type, 
the  crossing  span  and  the  next  adjoining  spans  shall  be  dead-ended 
at  the  poles,  or  towers,  supporting  the  crossing  span,  so  that  at  these 
poles,  or  towers,  the  insulators  shall  be  used  as  strain  insulators,  or 
the  height  of  the  wire  attachments  shall  be  such  that  with  the  maxi- 
mum sag  in  the  crossing  span,  occurring  from  failure  of  the  construc- 
tion outside  the  crossing  span,  and  taking  into  account  the  deflections 
in  the  strings  of  suspension  insulators,  the  minimum  clearances,  as 
given  in  Paragraphs  9  and  10,  shall  be  maintained. 

13.  The  clearance  in  any  direction  between  the  conductors  nearest 
the  pole,  or  tower  and  the  pole,  or  tower,  shall  be  not  less  than: 

Line  voltage  Clearances 

Not  exceeding  10,000  volts , .  '         9  in. 

Exceeding  10,000,  but  not  exceeding  14,000 12  in. 

Exceeding  14,000,  but  not  exceeding  27,000 15  in. 

Exceeding  27,000,  but  not  exceeding  35,000 18  in. 

Exceeding  35,000,  but  not  exceeding  47,000 21  in. 

Exceeding  47,000,  but  not  exceeding  70,000 24  in. 

1  NOTE. — This  requirement  does  not  apply  to  wires  of  the  same  phase 
or  polarity  between  which  there  is  no  difference  of  potential. 


250  POLE  AND  TOWER  LINES 

14.  Conductors. — The  normal  mechanical  tension  in  the  conductors 
generally  shall  be  the  same  in  the  crossing  span  and  in  the  adjoining 
span  on  each  side. 

15.  The  conductors  shall  not  be  spliced  in  the  crossing  span  nor 
in  the  adjoining  span  on  either  side. 

Taps  to  conductors  in  the  crossing  span  are  generally  objectionable, 
and  should  not  be  made  unless  necessary. 

16.  The  ties  or  devices  for  supporting  the  conductors  at  the  poles, 
or  towers,  shall  be  such  as  to  hold  the  wires,  under  maximum  loading, 
to  the  supporting  structures,  in  case  of  shattered  insulators,  or  wires 
broken  or  burned  at  an  insulator,  without  allowing  an  amount  of  slip 
which  would  materially  reduce  the  clearance  specified  in  Paragraphs  9 
and  10. 

17.  Ground  wires  when  installed   as  protection  against  lightning, 
shall  be  thoroughly  grounded  at  each  of  the  crossing  supports.     In 
case  of  their  installation  on  steel  supporting  structures,  they  may  be 
clamped  thereto.     In  case  they  are  installed  on  wooden  structures, 
the  ground  wire  shall  be  grounded  at  each  of  the  structures  with  a  solid 
copper  wire,  with  as  few  bends  as  possible,  and  no  sharp  bends,  and  not 
less  than  No.  4  B.  &  S.  gage   or  equivalent    copper  section.     The 
ground  wire  itself,  in  the  crossing  span  and  the  adjacent  spans,  may  be 
of  the  same  material  as  the  conductors,  or  a  steel  strand  not  less  than 
5/16  in.  in  diameter  may  be  used,  double  galvanized,  and  having  a 
breaking  strength  of  not  less  than  4500  Ib.  and  in  general  shall  follow 
the  minimum  factors  of  safety  as  provided  for  the  rest  of  the  crossing 
construction. 

If  crossarms  are  grounded,  the  same  ground  wire  may  be  used  for 
grounding  the  lightning  protection  wire  as  in  grounding  crossarm  strips. 

18.  Where  there  is  an  upward  stress  at  the  point  of  conductor  attach- 
ment, the  attachment  shall  be  of  such  type  as  to  properly  hold  the 
conductor  in  place. 

19.  Guys. — Wooden  poles  supporting  the  crossing  span  shall  be  side- 
guyed  in  both  directions,  if  practicable,  and  be  head-guyed  away  from 
the  crossing  span,  and  the  next  adjoining  poles  shall  be  head-guyed 
toward  the  crossing  span.     Braces  may  be  used  instead  of  guys. 

20.  Strain  insulators  shall  be  used  in  guys  from  wooden  poles,  except 
when  the  guys  are  through  grounded  to  permanently  damp  earth. 

The  insulators  shall  be  placed  not  less  than  eight  (8)  ft.  from  the 
ground.  Strain  insulators  shall  not  be  used  in  guying  steel  poles  or 
structures. 

21.  Clearing. — The  space  around  the  poles,  or  towers,  shall  be  kept 
free  from  inflammable  material,  underbrush  and  grass. 

22.  Signs. — In  the  case  of  railroad  crossings,  if  required  by  the  rail- 
road company,  warning  signs  of  an  approved  design  shall  be  placed  on 
all  poles  and  towers  located  on  the  railroad  company's  right-of-way. 


OVERHEAD  CROSSINGS  251 

23.  Grounding. — For  voltages  over  5000  volts,  wooden  crossarms,  if 
used,  shall  be  provided  with  a  grounded  metallic  plate  on  top  of  the 
arm  which  shall  be  not  less  than  %  in.  in  thickness  and  which  shall 
have  a  sectional  area  and  conductivity  not  less  than  that  of  the  line 
conductor.    Metal  pins  shall  be  electrically  connected  to  this  ground. 
Metal  poles  and  metal  arms  on  wooden  poles  shall  be  grounded. 

24.  The  electrical  conductivity  of  the  ground  conductor  shall  be 
adjusted  to  the  short-circuit  current  capacity  of  the  system  at  the 
crossing  and  shall  be  not  less  than  that  of  a  No.  4  B.  &  S.  gage  copper 
wire. 

25.  Temperature. — In  the  computation    of  stresses  and  clearances 
and  in  erection,  provision  shall  be  made  for  a  variation  in  tempera- 
ture from  -  20°F.  to  +  120°F.    A  suitable  modification  in  the  tem- 
perature requirements  shall  be  made  for  regions  in  which  the  above 
limits  would  not  fairly  represent  the  extreme  range  of  temperature. 

26.  Inspection. — If  required  by  contract,  all  material  and  workman- 
ship shall  be  subject  to  the  inspection  of  the  company  crossed;  pro- 
vided that  reasonable  notice  of  the  intention  to  make  shop  inspection 
shall  be  given  by  such  company.     Defective  material  shall  be  rejected 
and  shall  be  removed  and  replaced  with  suitable  material. 

27.  On  the  completion  of  the  work,  all  false  work,  plant  and  rubbish 
incident  to  the  construction  shall  be  removed  promptly  and  the  site 
left  unobstructed  and  clean. 

28.  Drawings. — If  required,  by  contract, 

( )  complete  sets  of  general  and  detail  drawings  shall  be  fur- 
nished for  approval  before  any  construction  is  commenced. 

LOADS 

29.  The  conductors  shall  be  considered  as  uniformly  loaded  through- 
out their  length,  with  a  load  equal  to  the  resultant  of  the  dead  load  plus 
the  weight  of  a  layer  of  ice  Y2  in.  in  thickness  and  a  wind  pressure  of 
8  Ib.  per  square  foot  on  the  ice-covered  diameter,  at  a  temperature  of 
0°F. 

30.  The  weight  of  ice  shall  be  assumed  as  57  Ib.  per  cubic  foot  (0.033 
Ib.  per  cubic  inch). 

31.  Insulators,  pins  and  conductor  attachments  shall  be  designed  to 
withstand  the  mechanical  tension  in  the  conductors  under  the  maximum 
loadings  with  the  designated  factor  of  safety. 

32.  Sags  should  be  such  that  the  stress  on  the  pin  falls  within  the 
limits  of   paragraph  31,  unless  methods  be  employed  to  prevent  an 
undue  slip  in  case  of  pin  failure.     (See  paragraphs  9,  10  and  16.) 

33.  The  pole,  or  towers,  shall  be  designed  to  withstand,  with  the 
designated  factor  of  safety,  the  combined  stresses  from  their  own 
weight,  the  wind  pressure  on  the  pole,  or  tower  and  the  above  wire 


252  POLE  AND  TOWER  LINES 

loading  on  the  crossing  span  and  the  next  adjoining  span  on  each 
side.  The  wind  pressure  on  the  poles,  or  towers,  shall  be  assumed 
at  13  Ib.  per  square  foot  on  the  projected  area  of  solid  or  closed  struc- 
tures and  one  and  one-half  times  the  projected  area  of  latticed 
structures. 

34.  The  poles,  or  towers,  shall  also  be  designed  to  withstand  the  loads 
specified  in  paragraph  33,  combined  with  the  unbalanced  tension  of: 

2  broken  wires  for  poles,  or  towers,  carrying  5  wires  or  less. 

3  broken  wires  for  poles,  or  towers,  carrying  6  to  10  wires. 

4  broken  wires  for  poles,  or  towers,  carrying  11  or  more  wires. 

35.  Crossarms  shall  be  designed  to  withstand  the  loading  specified 
in   paragraph  33,  combined  with  the  unbalanced  tension  of  one  wire 
broken  at  the  pin  farthest  from  the  pole. 

36.  The  poles,  or  towers,  may  be  permitted  a  reasonable  deflection 
under  the  specified  loading,  provided  that  such  deflection  does  not 
reduce  the  clearance  specified  in  paragraph  10  more  than  twenty-five 
(25)  per  cent,  or  produce  stresses  in  excess  of  those  specified  in  para- 
graphs 69  to  73. 

FACTORS  OF  SAFETY 

37.  The  ultimate  unit  stress  divided  by  the  allowable  unit  stress  shall 
be  not  less  than  the  following: 

Wires  and  cables 2 

Pins 2 

Insulators,  conductor  attachments,  guys 3 

Wooden  poles  and  crossarms 6 

Structural  steel 3 

Reinf orced-concrete  poles  and  crossarms 4 

Foundations 2 

NOTE. — The  use  of  treated  wooden  poles  and  crossarms  is  recommended. 
The  treatment  of  wooden  poles  and  crossarms  should  be  by  thorough 
impregnation  with  preservative  by  either  closed  or  open-tank  process.  For 
poles,  except  in  the  case  of  yellow  pine  the  treatment  need  not  extend  higher 
than  a  point  2  ft.  above  the  ground  line. 

38.  Insulators. — Insulators  for  line  voltages  of  less  than  9000  shall 
not  flash  over  at  four  times  the  normal  working  voltage,  under  a  pre- 
cipitation of  water  of  y$  in.  per  minute,  at  an  inclination  of  45°  to 
the  axis  of  the  insulator. 

39.  Each  separate  part  of  a  built-up  insulator,  for  line  voltages  over 
9000,  shall  be  subjected  to  the  dry  flash-over  test  of  that  part  for  five 
consecutive  minutes. 


OVERHEAD  CROSSINGS  253 

40.  Each  assembled  and  cemented  insulator  shall  be  subjected  to  its 
dry  flash-over  test  for  five  consecutive  minutes. 

The  dry  flash-over  test  shall  be  not  less  than: 

Line  voltage  Test  voltage 

Exceeding    9,000  but  not  exceeding  14,000. . . .  65,000 

Exceeding  14,000  but  not  exceeding  27,000. . . .  100,000 

Exceeding  27,000  but  not  exceeding  35,000. . . .  125,000 

Exceeding  35,000  but  not  exceeding  47,000. . . .  150,000 

Exceeding  47,000  but  not  exceeding  60,000. . . .  180,000 

Exceeding  60,000 3  times  line  voltage 

Each  insulator  shall  further  be  so  designed  that,  with  excessive 
potential,  failure  will  first  occur  by  flash-over  and  not  by  puncture. 

41.  Each  assembled  insulator  shall  be  subjected  to  a  wet  flash-over 
test,  under  a  precipitation  of  water  of  ^  in.  per  minute,  at  an  inclina- 
tion of  45°  to  the  axis  of  the  insulator. 

The  wet  flash-over  test  shall  be  not  less  than: 

Line  voltage  Test  voltage 

Exceeding    9,000  but  not  exceeding  14,000. . . .  40,000 

Exceeding  14,000  but  not  exceeding  27,000. . . .  60,000 

Exceeding  27,000  but  not  exceeding  35,000. . . .  80,000 

Exceeding  35,000  but  not  exceeding  47,000. . . .  100,000 

Exceeding  47,000  but  not  exceeding  60,000 120,000 

Exceeding  60,000 twice  the  line  voltage 

42.  Test  voltages  above  35,000  volts  shall  be  determined  by  the 
A.I.E.E.  Standard  Spark-gap  Method. 

43.  Test  voltages  below  35,000  yolts  shall  be  determined  by  trans- 
former ratio. 

MATERIAL 

44.  Conductors. — The  conductors  shall  be  of  copper,  aluminum,  or 
other  non-corrodible  material,  except  that  in  exceptionally  long  spans, 
where  the  required  mechanical  strength  cannot  be  obtained  with  the 
above  materials,  galvanized  or  copper-covered  steel  strand  may  be  used. 

45.  For  voltages  not  exceeding  750  volts,  solid  or  stranded  conductors 
may  be  used  up  to  and  including  0000  in  size;  above  0000  in   size, 
stranded  conductors  shall  be  used.     For  voltages  exceeding  750  volts 
and  not  exceeding   5000  volts,  solid  or  stranded  conductors  may  be 
used  up  to  and  including  00  in  size;  above  00  in  size,  conductors 
shall  be  stranded.     For  voltages  exceeding  5000  volts,  all  conductors 
shall  be  stranded.    Aluminum  conductors  for  all  voltages  and  sizes 
shall  be  stranded. 


254  POLE  AND  TOWER  LINES 

The  minimum  size  of  conductors  shall  be  as  follows: 
No.  6  B.  &  S.  gage  copper  for  voltages  not  exceeding  5000  volts. 
No.  4  B.  &  S.  gage  copper  for  voltages  exceeding  5000  volts. 
No.  1  B.  &  S.  gage  aluminum  for  all  voltages. 

46.  Insulators. — Insulators  shall  be  of  porcelain  for  voltages  exceed- 
ing 5000  volts. 

47.  For  pin  type  insulators,  there  shall  be  a  bearing  contact  between 
the  pin  and  the  insulator  pin  hole  up  to  the  level  of  the  top  of  the  tie 
wire  groove,  the  purpose  being  that  the  pin  should  directly  take  the 
strain  imposed  upon  the  insulator. 

48.  Strain  insulators  for  guys  shall  have  an  ultimate  strength  of 
not  less    than  twice    that    of    the    guy    in    which    placed.       Strain 
insulators  shall  be  so  constructed  that  the  guy  wires  holding  the 
insulator  in  position  will  interlock  in  case  of  the  failure  of  the  insulator. 

For  less  than  5000  volts,  strain  insulators  for  guys  shall  not  flash 
over  at  four  times  the  maximum  line  voltage  under  a  precipitation  of 
water  of  one-fifth  of  an  inch  (3/g  in.)  per  minute,  at  an  inclination  of 
45°  to  the  axis  of  the  insulator.  For  voltages  of  more  than  5000 
volts,  the  strain  insulator  or  series  of  strain  insulators  shall  not  fail  at 
the  line  voltage  under  the  above  precipitation  conditions. 

49.  Pins. — For  voltages  of  5000  and  over,  insulator  pins  shall  be  of 
steel,  wrought  iron,  malleable  iron,  or  other  approved  metal  or  alloy, 
and  shall  be  galvanized,  or  otherwise  protected  from  corrosion.     (See 
paragraph  47.) 

50.  Guys. — Guys   shall  be  galvanized   or  copper-covered  stranded 
steel  cable   not   less  than   %6  in.  in  diameter,  or   galvanized  rolled 
rods,  neither  to  have  an  ultimate  tensile  strength  of  less  than  4500  Ib. 

51.  Guys  to  the  ground  shall  connect  to  a  galvanized  anchor  rod, 
extending  at  least  1  ft.  above  the  ground  level. 

52.  The  detail  of  the  anchorage  shall  be  definitely  shown  upon  the 
plans. 

53.  Wooden  Poles. — Wooden   poles    shall    be    of   selected    timber, 
reasonably  straight,  peeled,  free  from  defects  which  would  decrease 
their  strength  or  durability,  not  less  than  8  in.  in  diameter  at  the  top, 
and  meeting  the  requirements  as  specified  in  paragraphs  19,  33,  34 
and  37. 

54.  Concrete. — All  concrete  and  concrete  material  shall  be  in  accord- 
ance with  the  requirements  of  the  Report  of  the  Joint  Committee  on 
Concrete  and  Reinforced  Concrete.1 

STRUCTURAL  STEEL 

55.  Structural  steel  shall  be  in  accordance  with  the  Manufacturers' 
Standard  Specifications. 

1NoTE. — This  may  be  found  in  the  February,  1913,  Proceedings  of  the 
American  Society  of  Civil  Engineers,  Vol.  59,  No.  2,  pp.  117-168. 


OVERHEAD  CROSSINGS  255 

56.  The  design  and  workmanship  shall  be  strictly  in  accordance  with 
first-class  practice. 

57.  The  form  of  the  frame  shall  be  such  that  the  stresses  may  be 
computed  with  reasonable  accuracy,  or  the  strength  shall  be  deter- 
mined by  actual  test. 

58.  The  sections  used  shall  permit  inspection,  cleaning  and  painting, 
and  shall  be  free  from  pockets  in  which  water  or  dirt  can  collect. 

59.  The  length  of  a  main  compression  member  shall  not  exceed  180 
times  its  least  radius  of  gyration.     The  length  of  a  secondary  compres- 
sion member  shall  not  exceed  220  times  its  least  radius  of  gyration. 

60.  The  minimum  thickness  of  metal  in  galvanized  structures  shall 
be  y±  in.  for  main  members  and  ^  in.  for  secondary  members.     The 
minimum  thickness  of  painted  material  shall  be  y±  in. 

PROTECTIVE  COATINGS 

61.  All  structural  steel  shall  be  thoroughly  cleaned  at  the  shop  and 
be  galvanized,  or  given  one  coat  of  approved  paint. 

62.  Painted  Materials. — All  contact  surfaces  shall  be  given  one  coat 
of  paint  before  assembling. 

All  painted  structural  steel  shall  be  given  two  field  coats  of  an  ap- 
proved paint. 

The  surface  of  the  metal  shall  be  thoroughly  cleaned  of  all  dirt,  grease, 
scale,  etc.,  before  painting  and  no  painting  shall  be  done  in  freezing 
or  rainy  weather. 

63.  Galvanized  Material. — Galvanized  material  shall  be  in  accord- 
ance with  the  Specification  for  Galvanizing  Iron  and  Steel. 

Bolt  holes  in  galvanized  material  shall  be  made  before  galvanizing. 
Sherardizing  for  small  parts  is  permissible. 

FOUNDATIONS 

64.  The  foundations  for  steel  poles  and  towers  shall  be  designed  to 
prevent  overturning. 

The  weight  of  concrete  shall  be  assumed  as  140  Ib.  per  cubic  foot. 
In  good  ground,  the  weight  of  "earth"  (calculated  at  30°  from 
the  vertical)  shall  be  assumed  as  100  Ib.  per  cubic  foot.  In  swampy 
ground,  special  measures  shall  be  taken  to  prevent  uplift  or  depression. 

Concrete  for  foundation  shall  be  well  worked,  very  wet,  and  shall 
not  be  leaner  than  one  part  Portland  cement,  three  parts  clean,  sharp 
sand,  and  six  parts  of  broken  stone,  or  one  part  Portland  cement  to 
six  parts  of  good  gravel,  free  from  loam  or  clay. 

65.  The  top  of  the  concrete  foundation,  or  casing,  shall  be  not  less 
than  six  (6)  in.  above  the  surface  of  the  ground,  nor  less  than  one 
(1)  ft.  above  high  water,  except  that  no  foundation  need  be  higher  than 


256 


POLE  AND  TOWER  LINES 


the  base  of  the  railroad  company's   rail,  or  the  top  of  the    traveled 
roadway. 

66.  When  located  in  swampy  ground,    wooden    crossing    and   next 
adjoining  poles  shall  be  set  in  barrels  of  broken  stone  or  gravel,  or 
in  broken  stone  or  timber  footings. 

67.  When  located  in  the  sides  of  banks,  or  when  subject  to  wash- 
outs, foundations  shall  be  given  additional  depth,  or  be  protected  by 
cribbing  or  riprap. 

68.  All  foundations  and  pole  settings  shall  be  tamped  in  six  (6) 
in.  layers,  while   backfilling.     It  is  desirable  in  backfilling  that  the 
earth  be  suitably  moistened. 

WORKING  UNIT  STRESSES 

Obtained  by  dividing  the  ultimate  breaking  strength  by  the  factors 
of  safety  given  in  paragraph  37. 

69.  Structural  Steel: 

Lb.  per  sq.  in. 

Tension  (net  section) 18,000 

Shear 14,000 

Compression 18,000  -  60- 

70.  Rivets,  Pins:  Lb.  per  8q.  in 

Shear 10,000 

Bearing 20,000 

Bending 20,000 

71.  Bolts:  Lb.  persq.in. 

Shear 8,500 

Bearing. 17,000 

Bending 17,000 

72.  Wires  and  Cables: 

Lb.  per  sq.  in 

Copper,  hard-drawn,  solid,  B.  &  S.  gage  0000,  000,  00  25,000 

Copper,  hard-drawn,  solid,  B.  &  S.  gage  0 27,500 

Copper,  hard-drawn,  solid,  B.  &  S.  gage  No.  1 28,500 

Copper,  hard-drawn,  solid,  B.  &  S.  gage  Nos.  2,  4,  6.  30,000 

Copper,  soft-drawn,  solid 17,000 

Copper,  hard-drawn,  stranded 30,000 

Aluminum,  hard-drawn,  stranded,  B.  &  S.  gage  under 

0000 12,000 

Aluminum,  hard-drawn,  stranded,  B.  &  S.  gage  No. 

0000  and  over 11,500 


OVERHEAD  CROSSINGS  257 

73.   Untreated  Timber: 

Lb.persq.in.  1-^ 

Eastern  white  cedar 600  600 

Chestnut 850  850 

Washington  cedar 850  850 

Idaho  cedar 850  850 

Port  Orford  cedar 1150  1150 

Long-leaf  yellow  pine 1000  1000 

Short-leaf  yellow  pine 800  800 

Douglas  fir 900  900 

White  oak 950  950 

Red  cedar 700  700 

Bald  cypress  (heartwood) 800  800 

Redwood 650  650 

Catalpa 500  500 

Juniper 550  550 

L  =  Length  in  inches. 

D  =  Least  side,  or  diameter,  in  inches. 


17 


GENERAL  SPECIFICATIONS  FOR  ELECTRIC  LIGHT  AND 
POWER  LINES 

BY  R.  D.  COOMBS 

CLEARANCES 

1.  Conductors. — The  clear  headroom  above  a  highway,  under  the  most 
unfavorable  condition  of  temperature  and  loading,  shall  be  not  less  than 
20ft. 

2.  The  vertical  overhead  clearance  from  any  telephone  or  similar 
wire  on  the  power  line,  shall  be  not  less  than: 


Line  voltage 

Not  exceeding  6,600  volts 

Exceeding  6,600  but  not  exceeding 
22,000  but  not  exceeding 
45,000  but  not  exceeding 
66,000  but  not  exceeding 


Exceeding 
Exceeding 
Exceeding 
Exceeding 


22,000 
45,000 
66,000 
88,000. 


88,000  but  not  exceeding  110,000. 


Exceeding  110,000 


Clearance 
2ft. 
4ft. 
5ft. 
6ft. 
7ft. 
8ft. 
10  ft. 


.9  5 


,01     234     5    6     78     9    10  11  12  13  14  15  16  17  18  19  20 
Sag  in  Feet 

FIG.  1. — Conductor  separations. 

3.  The  separation  of  alternating-current  conductors  on  pin-type 
insulators  in  a  horizontal  plane  shall  be  in  general  not  less  than  that 
required  for  the  sag  in  question  in  Fig.  1  nor  less  than: 

258 


GENERAL  SPECIFICATIONS  259 

Line  voltage  Spacing 

Not  exceeding  6,600  volts 12  in. 

Exceeding    6,600  but  not  exceeding  13,000 18  in. 

Exceeding  13,000  but  not  exceeding  22,000 24  in. 

Exceeding  22,000  but  not  exceeding  33,000 30  in. 

Exceeding  33,000  but  not  exceeding  45,000 40  in. 

Exceeding  45,000  but  not  exceeding  66,000 50  in. 

Exceeding  66,000  but  not  exceeding  88,000 60  in. 

4.  The    separation    of    conductors    supported    by    suspension  type 
insulators,  in  a  horizontal  plane,  shall  be  that  required  in  paragraph 
3,  plus  one  and  one-quarter  times  the  length  of  the  suspension  string. 

5.  The  clearance  between  a  conductor  and  any  part  of  the  structure 
shall  be  not  less  than: 

Line  voltage  Clearance 

Not  exceeding  6,600  volts 9  in. 

Exceeding    6,600  but  not  exceeding  13,000 12  in. 

Exceeding  13,000  but  not  exceeding  22,000 15  in. 

Exceeding  22,000  but  not  exceeding  33,000 ". .  18  in. 

Exceeding  33,000  but  not  exceeding  45,000 21  in. 

Exceeding  45,000  but  not  exceeding  66,000 24  in. 

Exceeding  66,000  but  not  exceeding  88,000 27  in. 

Exceeding  88,000  but  not  exceeding  110,000 29  in. 

Exceeding  110,000 30  in. 

NOTE. — This  requirement  does  not  apply  to  the  distance  between  the 
crossarms  for  an  insulator  of  the  through-phi  type. 

6.  The  side  clearance  between  a  conductor  supported  by  suspension 
insulators  and  the  supporting  structure  when  the  insulator  string  is 
deflected  45°,  shall  be  not  less  than  that  specified  in  paragraph  5. 

7.  Ground  Wire. — The  longitudinal  overhead  ground  wire  or  wires 
shall  be  in  general  not  more  than  45°  from  the  vertical  through  the 
adjoining  conductor,  and  with  a  separation  of  not  less  than  that  required 
by  the  table  in  paragraph  3. 

LOADS  AND  FACTORS  OF  SAFETY 

8.  Ice  and  Wind  Loads. — The  conductors  shall  be  considered  as  uni- 
formly loaded  throughout  their  length,  with  a  load  equal  to  the  resultant 
of  the  dead  load  plus  the  weight  of  a  layer  of  ice  %  in.  in  thickness 
and  a  wind  pressure  of  8  Ib.  per  square  foot  on  the  ice-covered  diameter, 
at  a  temperature  of  0°F. 

9.  The  weight  of  ice  shall  be  assumed  as  57  Ib.  per  cubic  foot  (0.033 
Ib.  per  cubic  inch). 

10.  The  wind  pressure  on  poles  or  towers  shall  be  assumed  at  8  Ib. 
per  square  foot,  on  the  projected  area  of  solid  or  closed  poles,  and  on  one 
and  one-half  times  the  projected  area  of  latticed  poles,  and  on  twice 
the  projected  area  of  wide  base  structures. 


260  POLE  AND  TOWER  LINES 

In  regions  in  which  there  is  no  sleet,  the  ice  load  may  be  omitted  and 
the  wind  pressure  increased  to  15  Ib.  per  square  foot. 

11.  Conductors  and  Ground  Wires. — For  spans  exceeding  150  ft.  and 
for  lines  not  on  streets,  the  conductors  and  overhead  ground  wires  shall 
be  designed  to  withstand  the  above  ice  and  wind  loads,  without  exceed- 
ing the  elastic  limit  of  the  material. 

12.  For  spans  not  exceeding  150  ft.  and  for  all  lines  on  streets,  the 
above  loading  may  be  reduced  25  per  cent. 

13.  Ground-wire  Connections. — The  ground  wire  connections  shall  be 
designed  to  withstand  the  maximum  stress  in  the  ground  wire,  without 
exceeding  the  elastic  limit  of  the  material. 

14.  Supports. — For  spans  exceeding  150  ft.  and  for  lines  not  on  streets 
the  poles  or  towers  shall  be  designed  to  withstand  the  combined  stresses 
from  their  own  weight,  the  wind  pressure  on  the  structure  and  the  above 
wire  loading  on  the  adjoining  spans  combined  with  the  effect  of  one 
broken  conductor,  with  a  factor  of  safety  of  2.0. 

15.  For  spans  not  exceeding  150  ft.  and  all  lines  on  streets,  the  sup- 
porting structures  shall  be  designed  to  withstand  the  above  loading. 

NOTE. — Guys  may  be  used  to  obtain  the  strength  required  by  para- 
graphs 14  and  15. 

16.  Insulators  and  Pins. — Insulators  and  pins  at  corners  or  bends  in 
the  line  shall  be  designed  to  withstand  the  transverse  loads  resulting 
from  the  above  ice  and  wind  loads  on  the  conductors,  combined  with  the 
horizontal  component  due  to  the  tension  in  the  wires  and  the  angle  in 
the  line,  with  a  factor  of  safety  of  one  and  one-half  (1.5). 

NOTE — Double  arms  may  be  used  to  obtain  the  requisite  strength. 

17.  Suspension  Type   Insulators. — Suspension    type   insulators  and 
their  connections  shall  be  designed  to  withstand  the  maximum  tension  in 
the  conductors,  with  a  factor  of  safety  of  one  and  one-half  (1.5)  when 
used  in  the  suspension  position  and  2.0  when  used  in  the  strain  position. 

18.  Guy  Insulators. — Strain  insulators  for  guys  shall  be  designed  to 
withstand  the  maximum  stress  in  the  guy,  with  a  factor  of  safety  of 
two  (2). 

19.  Guys. — Guys  shall  be  designed  to  withstand  their  maximum  stress 
with  a  factor  of  safety  of  two  (2). 

20.  Guy  Anchorages. — Guy  anchorages  shall  be  designed  to  withstand 
the  maximum  stress  in  the  guys  with  a  factor  of  safety  of  one  and  one- 
half  (1.5). 

21.  Foundations. — The  foundations  of  unguyed  poles  and  towers  shall 
be  designed  to  resist  overturning,  with  a  factor  of  safety  of  two  (2) . 

22.  Temperature. — In  the  computation  of  stresses  and  clearances  and 
in  erection,  provision  shall  be  made  for  a  variation  in  temperature  from 
—  20°F.  to  120°F.    A  suitable  modification  in  the  temperature  require- 


GENERAL  SPECIFICATIONS-  261 

ments  may  be  made  for  regions  in  which  the  above  limits  would  not 
fairly  represent  the  extreme  range  of  temperature. 

23.  Guys   or  Special  Supports. — Guys,  or  supporting  structures  of 
greater  strength  than  required  to  withstand  the  preceding  loads,  shall 
be  installed  approximately  as  follows: 

Wooden  poles — 

Side    guy — all  bends  or  corners. 

Head  guy — steep  hills. 

Head  guy — unusually  long  spans. 
Head  and  side  guy — light  lines  every  1500  ft. 
Head  and  side  guy — heavy  lines  every  1000  ft. 
Flexible  structures — 

Side    guy — corners  over  5°. 

Head  guy — steep  hills. 

Head  guy — unusually  long  spans. 

Head  guy — or  special  structure,  every  2000  ft. 

Steel  or  concrete  poles — 

Side    guy — sharp  corners. 
Head  guy — steep  hills. 
Head  guy — unusually  long  spans. 
Head  guy — light  lines  every  3000  ft. 
Head  guy — heavy  lines  every  2000  ft. 
Rigid  towers — 

Special  structure — sharp  corners. 
Special  structure — every  mile. 

MATERIAL 

24.  Overhead  Ground  Wire. — The  material  of  ground  wires  shall  be 
copper,  copper-covered  steel,  galvanized  iron,  galvanized  steel  or  an 
approved  alloy;  sizes  over  No.  4  B.  &  S.  gage  shall  be  stranded. 

NOTE. — The  use  of  galvanized  steel  ground  wire  is  not  recommended 
except  in  sizes  %  in.  or  more  in  diameter. 

25.  The  attachment  of  the  ground  wire  to  the  structure  shall  be  by 
means  of  a  smooth  grip  with  well-rounded  ends,  and  a  contact  length  of 
not  less  than  three  inches  (3  in.). 

26.  Conductors. — Conductors  shall  be  of  copper,  aluminum  or  other 
approved  material. 

27.  Insulators. — Insulators  shall  be  of  porcelain,  glass  or  other  ap- 
proved material. 

28.  Wooden  Pins. — Wooden   pins  shall  be  sound,  straight  grained 
yellow  or  black  locust  or  other  approved  species,  free  from  knots  over 
Y%  in.  in  diameter,  except  on  the  shoulder  or  lower  half  of  the  shank, 
and  free  from  checks,  sapwood  and  worm  holes. 

29.  Metal  Pins. — For  voltages  over  13,000,  insulator  pins  shall  be  of 


262  POLE  AND  TOWER  LINES 

steel,  wrought  iron,  malleable  iron,  an  approved  metal  or  alloy  or  a 
combination  of  steel  with  wood,  metal  or  porcelain. 

NOTE. — Wood  pins  may  be  used  for  higher  voltages  in  regions  having 
favorable  climatic  conditions. 

30.  Wooden    Crossarms. — Wooden    crossarms   shall   be  of    seasoned 
timber,  reasonably  straight  grained,  out  of  wind,  free  from  large,  loose 
or  unsound  knots,  wane,  large  pitch  pockets,  pitch  pockets  which  enter 
the  pin  or  bolt  holes,  through  shakes,  shakes  or  checks  over  3  in.  long 
and  rot  or  worm  holes. 

31.  Wooden  Poles. — Wooden  poles  shall  be  of  approved  species  of 
timber,  peeled,  with  trimmed  knots,  reasonably  straight,  well  propor- 
tioned from  butt  to  tip,  with  squared  ends,  free  from  defects  which  would 
materially  decrease  their  strength  or  durability  and  of  not  less  than  7- 
in.  minimum  diameter  at  the  top. 

32.  Guys. — Guys  shall  be  stranded,  galvanized  steel  or  copper  covered 
cable,  not  less  than  ^6  in.  in  diameter. 

33.  Pole  Steps. — Pole  steps  shall  be  of  forged  or  rolled  iron  or  steel,  in 
accordance  with  the  Manufacturers'  Standard  Specification. 

34.  Steel. — Structural  steel  work  shall  be  of  open-hearth  steel,  in 
accordance  with  the  Manufacturers'  Standard  Specification. 

STRUCTURAL  DESIGN 

35.  Frame. — The  form  of  the  frame  shall  be  such  that  the  stresses  may 
be  computed  with  reasonable  accuracy. 

36.  The  sections  used  shall  permit  inspection,  cleaning  and  painting 
and  shall  be  free  from  pockets  in  which  water  or  dirt  can  collect. 

37.  The  length  of  a  main  compression  member  shall  not  exceed  125 
times  its  least  radius  of  gyration.     The  length  of  a  secondary  compres- 
sion member  shall  not  exceed  180  times  its  least  radius  of  gyration. 

38.  Minimum  Sections  and  Connections. — The  minimum  thickness  of 
metal  shall  be  one-quarter  inch  (y±  in.)  for  main  members  and  three- 
sixteenth  inch  (%6  in.)  for  secondary  members. 

In  wide-base  structures  the  minimum  angle  shall  be  not  less  than 
l;Hj  X  1^£  X  %6  m-»  and  the  minimum  main  bracing  connections 
shall  be  two  bolts. 

39.  Rivets  and  Bolts. — The  minimum  diameter  of  rivets  and  bolts  shall 
be  one-half  inch  (^  in.). 

40.  The  diameter  of  a  rivet  or  bolt  hole  shall  not  exceed  the  diameter 
of  the  rivet  or  bolt  by  more  than  one-sixteenth  of  an  inch  (^6  in.). 

41.  The  distance  center  to  center  of  rivet  or  bolt  holes  shall  be  not 
less  than: 

Diameter  of  bolt  or  rivet  Spacing 

K  in.  IH  in. 

%  in.  1%  in. 

%  in.  23^  in. 

in.  2^  in. 


GENERAL  SPECIFICATIONS  263 

42.  End  and  Edge  Distances. — The  distance  from  the  center  of  a  bolt 
or  rivet  hole  to  a  rolled  edge  or  to  a  sheared  end  shall  be  not  less  than: 

Diameter  of  bolt  or  rivet  Edge  distance  End  distance 

H  in.  %  in.  %  in. 

%  in.  %  in.  %  in. 

%  in.  1      in.  1^  hi. 

K  in.  IH  in.  \Y±  in. 

43.  Rods. — The  minimum  diameter  of  rod  bracing  shall  be  one-half 
inch  (Yz  in.). 

44.  Main  diagonal  rod  bracing  shall  be  provided  with  adjustable  end 
connections  having  right  and  left  threads. 

PROTECTIVE  COATINGS 

45.  All  structural  steel  shall  be  thoroughly  cleaned  at  the  shop  and 
galvanized  or  given  one  coat  of  approved  paint. 

NOTE. — In  view  of  the  thin  sections  used  in  the  class  of  work  covered  by 
these  specifications,  the  cleaning  and  painting  or  galvanizing  required 
herein  will  be  rigidly  enforced.  The  make  and  brand  of  paint,  or  the 
mixture  to  be  used,  for  both  shop  and  field  coats,  shall  be  given  hi  a  written 
notification,  a  copy  of  which  may  be  furnished  the  paint  manufacturer. 

46.  Metal  pins  shall  be  galvanized  or  otherwise  protected  from  corro- 
sion. 

47.  Hardware. — All  bolts,  braces,  lag  screws,  washers,  etc.,  used  on 
wooden  or  reinforced-concrete  poles,  shall  be  galvanized  or  sherardized 
in  accordance  with  the  Standard  Specifications  for  galvanizing. 

48.  Guys  shall  be  galvanized  or  copper-covered. 

49.  If  required,  wooden  erossarms  shall  be  treated  with  an  approved 
preservative  or  given  two  coats  of  approved  paint. 

50.  Wooden  pole  tops,  crossarm  gains  and  bolt  holes  shall  be  treated 
with  paint  or  preservative. 

NOTE. — The  application  at  the  ground  line  of  at  least  a  double-brush 
treatment  with  preservative  is  recommended. 

51.  Painted  Material. — All  contact  surfaces  shall  be  given  one  coat  of 
paint  before  assembling. 

52.  All  painted  structural  steel  shall  be  given  one  field  coat  of  an 
approved  paint. 

53.  The  surface  of  the  metal  shall  be  thoroughly  cleaned  of  all  dirt, 
grease,  scale,  etc.,  before  painting,  and  no  painting  shall  be  done  in 
freezing  or  rainy  weather. 

54.  Galvanized  Material. — Galvanized  material  shall  be  in  accordance 
with  the  Standard  Specification  for  galvanizing. 

55.  The  spelter  material  shall  be  Prime  Western  for  structural  steel 
and  Grade  A  for  wire,  or  equal. 


264 


POLE  AND  TOWER  LINES 


FOUNDATIONS 

56.  Tn  swampy  or  otherwise  uncertain  ground,  the  line  supports 
shall  be  provided  with  broken  stone,  gravel,  concrete  or  timber  footings, 
and  when  located  in  the  sides  of  banks  or  subject  to  washouts,  shall  be 
given  additional  depth  or  protected  by  cribbing,  riprap,  etc. 

57.  When  possible,  the  top  of  a  concrete  foundation  or  casing  shall 
extend  not  less  than  six  inches  (6  in.)  above  the  ground  nor  less  than  one 
foot  (1  ft.)  above  high  water. 

58.  The  top  of  the  foundation  shall  slope  down  toward  the  sides,  and 
be  built  up  in  the  corner  of  angles,  etc.,  to  provide  efficient  drainage. 

59.  The  thickness  of  the  concrete  casing  around  the  butt  of  a  steel 
pole  shall  be  not  less  than  three  inches  (3  in.). 

60.  Wooden  Pole  Settings. — Poles  shall  be  set  in  the  ground  to  depths 
not  less  than  those  specified  in  the  following  table: 

DEPTH  OF  SETTING 


Total  length  of  pole 
(ft.) 

Straight  lines 
(ft.) 

Curves,  corners  and  points  of 
extra  strain  (ft.) 

30 

5.0 

6.0 

35 

5.5 

6.0 

40 

6.0 

6.5 

45 

6.5 

7.0 

50 

6.5 

7.0 

55 

7.0 

7.5 

60 

7.0 

7.5 

65 

7.5 

8.0 

70 

7.5 

8.0 

*75 

8.0 

8.5 

80 

8.0 

8.5 

61.  Steel  and  Concrete  Pole  Settings. — The  penetration  below  ground 
of  a  steel  or  concrete  pole  shall  be  not  less  than  given  in  paragraph  60, 
except  that  concrete  poles  or  steel  poles  of  greater  butt  diameter  than 
twenty   inches   (20  in.)   incased  in   concrete,   may  have  nine  inches 
(9  in.)  less  penetration  than  otherwise  provided. 

62.  Towers  or  Wide  Base  Structures. — Wide  base  towers  or  structures 
not  provided  with  a  web  system  below  ground,  shall  be  secured  in  a 
foundation  designed  to  resist  lateral  movement  at  the  ground  line, 
either  by  the  use  of  sufficient  superficial  area,  concrete  or  cribbing. 

63.  The  penetration  shall  be  not  less  than  six  feet  (6  ft.). 

64.  The  anchorage  plate  shall  be  designed  to  withstand  the  maximum 
stress  in  the  main  leg,  with  a  factor  of  safety  of  two  (2),  and  in  general 
shall  be  not  less  than  four  hundred  square  inches  (400  sq.  in.)  in  area. 


GENERAL  SPECIFICATIONS  265 

65.  Excavation. — The  bottom  of  the  excavation  shall  be  compacted 
and  if  required  shall  be  covered  with  a  rammed  layer  of  broken  stone  and 
sand,  or  gravel,  or  be  covered  with  concrete. 

CONCRETE 

66.  Cement— The  cement  shall    be  Portland,  and  shall  meet   the 
requirements  of  the  Standard  Specifications. 

67.  Aggregates. — Aggregates  shall  consist  of  sand,  gravel,  broken  stone 
or  other  approved  material,  graded  from  fine  to  coarse,  free  from  vege- 
table matter  and  soft  particles  and  reasonably  clean. 

68.  Water. — Water  used  in  mixing  concrete  shall  be  free  from  oil,  acid 
and  injurious  amounts  of  alkalies  or  vegetable  matter. 

69.  Proportions. — For  plain  concrete  or  mass  foundations,  not  less 
than  one  part  cement  to  a  total  or  nine  (9)  parts  of  fine  and  coarse 
aggregates,  measured  separately,  shall  be  used. 

70.  For  reinforced  concrete  not  less  than  one  (1)  part  of  cement  to  a 
total  of  six  (6)  parts  of  fine  and  coarse  aggregates,  measured  separately, 
shall  be  used. 

71.  Such  relative  amounts  of  fine  and  coarse  aggregates  shall  be  used 
as  will  produce  a  dense  uniform  concrete. 

72.  Mixing. — The  materials  shall  be  well  mixed,  using  sufficient  water 
to  form  a  mixture  wet  enough  to  flow  in  the  forms  and  about  the  rein- 
forced or  incased  metal,  but  which  will  not  permit  the  separation  of  the 
coarser  aggregates  from  the  mortar. 

73.  Workmanship. — Proper  precautions  shall  be  taken  to  prevent  the 
freezing  of  concrete. 

74.  Mortar  or  concrete  which  has  partially  set  shall  not  be  remixed 
and  used. 

75.  Exposed  surfaces  of  the  concrete  shall  be  rubbed  smooth  without 
plastering. 

76.  Exposed  edges  or  corners  shall  be  rounded  or  beveled. 

77.  Concrete  Poles. — Reinforced-concrete  poles  shall  be  made  strictly 
in  accordance  with  the  best  practice  in  workmanship,  using  approved 
aggregates  and  producing  a  concrete  of  great  density. 

78.  No  part  of  the  reinforcement  shall  be  less  than  one  inch  (1  in.) 
from  the  surface. 

79.  No  cast  metal  shall  be  used  in  the  skeleton  reinforcement. 

80.  The  reinforcing  rods  shall  be  capable  of  being  bent  cold  180° 
around  a  circle  of  four  diameters. 

81.  The  entire  surface  of  the  concrete  shall  be  rubbed  to  a  smooth 
finish  without  plastering. 

82.  Poles  shall  be  straight,  of  the  required  dimensions,  and  provided 
with  the  necessary  holes  for  crossarms,  brace  and  guy  bolts  and  sockets 
for  pole  steps. 


266  POLE  AND  TOWER  LINES 

TESTS 

83.  Insulators. — Each  insulator  shall  be  so  designed  that,  with  excess- 
ive potential,  failure  will  first  occur  by  flash-over  and  not  by  puncture. 

84.  Previous  to  the  electrical  tests  the  separate  parts  of  an  insulator 
shall  be  subject  to  inspection  for  mechanical  defects  in  material  or 
workmanship.     No  part  shall  contain  soft  porcelain,  crazing,  serious 
deformations  or  cracks  in  the  grooves  or  in  the  unglazed  portions  that 
would  materially  decrease  the  value  of  the  insulator. 

85.  The  assembled  insulators  shall  withstand  a  voltage  of  5000  volts 
less  than  specified  in  paragraphs  90  and  91,  for  five  consecutive  minutes, 
without  injury,  abnormal  static  strain,  noise,  or  flash-over. 

86.  Each  separate  part  of  a  built-up  insulator,  and  each  assembled 
and  cemented  insulator  shall  be  subjected  to  an  approved  factory  test. 

87.  The  wet  flash-over  test  shall  be  made  under  a  precipitation  of 
water  of  one-fifth  of  an  inch  per  minute,  at  an  angle  of  45°  to  the  axis 
of  the  insulator. 

88.  Test  voltages  above  35,000  volts  shall  be  determined  by  the 
A.I.E.E.  Standard  Spark-gap  Method. 

89.  Test  voltages  below  35,000  volts  shall  be  determined  by  trans- 
former ratio. 

90.  Pin  Insulators. — The  flash-over  design  test  voltage  shall  be  not 
less  than: 

Line  voltage  Dry  flash-over  Wet  flash-over 

Less  than  11,000  twice  line  voltage 

11,000  60,000  30,000 

22,000  90,000  50,000 

33,000  100,000  60,000 

45,000  125,000  90,000 

50,000  150,000  100,000 

60-70,000  180,000  120,000 

80,000  240,000  160,000 

91.  Suspension  Type  Insulators.- — The  flash-over  design  test  voltage 
of  the  string  of  suspension  units  shall  be  not  less  than: 

Line  voltage  Dry  flash-over  Wet  flash-over 

11,000  80,000  50,000 

22,000  160,000  90,000 

33,000  160,000  90,000 

45,000  220,000  130,000 

66,000  270,000  175,000 

88,000  310,000  220,000 

110,000  340,000  265,000 

125,000  460,000  300,000 

140,000  470,000  335,000 


GENERAL  SPECIFICATIONS  267 

92.  When  insulators  of  the  suspension  type  are  placed  in  the  strain 
position,  one  additional  insulator  unit  shall  be  used  in  series. 

93.  For  line  voltages  not  exceeding  9000  volts,  strain  insulators  for 
guys  shall  have  a  wet  flash-over  of  not  less  than  four  times  the  maximum 
line  voltage. 

94.  Reinforced-concrete  Poles. — If  required,  the  strength  of  concrete 
poles  shall  be  determined  by  testing  one  pole  from  each  lot  of 

95.  The  expense  of  testing  the  poles  which  withstand  the  specified 
conditions  of  loading  shall  be  borne  by  the  purchaser  and  the  expense 
of  testing  those  which  do  not  meet  the  requirements  shall  be  borne  by 
the  contractor. 

96.  Poles  shall  be  tested  by  applying  a  horizontal  pull  at  the  center 
of  gravity  of  the  wires,  and  continually  increasing  the  stress  until  it 
is  equivalent  to  the  specified  loading. 

97.  Test  poles  shall  be  set  in  a  firm  foundation. 

98.  The  test  load  shall  be  measured  by  a  dynamometer,  or  scale. 


INDEX 


Aerial  cable,  149 

duct  line,  150 
A  frames,  record  of,  132 

steel,  22,  23,  127  to  132 

wooden,  96, 
Aluminum,  45 
Anchors,  dead-man,  214 

holding  power,  215 

patent,  213 

Angles,  properties  of,  109 
Auxiliary  attachments,  240,  241 


Bearing,  rivets  and  bolts,  105 
Bi-metallic  wire,  44 
Bog  shoe,  166,  167 
Bolts,  104,  105 
Braces,  crossarm,  208,  209 
Bracing  (see  Lacing),  110 
Broken  wires  (see  Loading) 
Brush  treatment,  81,  82,  83 
Butt  treatment,  83 


C 


Cable,  properties  of,  54  to  57 

straightening,  217,  218 
Capacity,  of  line,  2  to  5 
Catenary,  47 
Cedar,  87 
Chestnut,  77,  86 
Clamps,  Crosby,  234 

crossing,  238  to  243 

ground  wire,  235 
Clearances,  6,  27,  236,  245,  248,  249, 

258,  259 
Coal  tar,  81,  82' 
Column  formulae,  68  to  73 
Concrete,  aggregates,  174 

cement,  173 


Concrete,  forms,  176 

mixing,  175 

proportions,  173 

reinforcement,  177 

specifications,  254,  265 

waterproofing,  178 

workmanship,  177 
Concrete  poles  (see  Poles) 
Copper,  43 
Costs,  general,  223  to  227 

steel  pole  line,  227,  228,  231 

steel  tower  line,  228 

wood-pole  line,  227,  230 
Cradles,  238,  248 
Creosote,  81,  82,  89 
Crossarm  braces  (see  Braces) 
Crossarms,  metal,  206 

wish-bone,  207 

wood,  205,  206,  207,  209 
Crossing  clamps,  238  to  243 

failures,  238 

river,  145  to  148 

specifications,  244,  248 


D 


Dead  man,  214 

Decay  and  defects,  77  to  80,  84 

Deflection  (see  Sag) 

Derrick  wagon,  219,  220 


Edge  and  end  distances,  bolts  and 

rivets,  106 
Erection,  217  to  231 

costs,  227  to  231 

derrick  wagon,  219,  220 

gin  pole,  222 

hauling,  224 

raising,  219,  222,  223,  225 

straightening  cable,  218 

stringing,  219 


269 


270 


INDEX 


discussion  of,  244 

L 

Lacing,  106,  117 


Factor  of  safety  16,  17,  18,  154,  193,      Joint  reP°rt  specification,  248 

245,  252 
Failures,  A-frames,  130,  131,  132 

crossings,  238 

towers,  165 
Flexibility,  21,  22,  129 
Flexible  frames  (see  A-frames) 
Foundations,  barrel,  169 

bog  shoe,  166,  167 

concrete,  169 

rock,  172 


rotten  rock,  172 
tower,  165,  171,  172 
Frames  (see  A-frames,  H-frames) 

G 

Galvanizing,  5,  182  to  188,  233 

Gin  pole,  222 

Grounding  arms,  98,  206,  239,  245 

poles,  239,  251 
Ground  wire,  232  to  236,  244,  250 

clamp,  235 
Guys  and  guying,  anchors,  213,  214 

general,  211,  212,  245,  250,  254, 
261 

insulators,  254 

tension,  213,  254 

wire,  211,  262 


Hardware  (see  Line  Material),  263 
H-frames,  97,  98 
High  towers,  145  to  148,  244 
House  derrick,  223 


Induction,  236 
Insulator  connections,  111 
Insulator  pins  (see  Pins) 
Insulators,  disc,  194 

guy,  195,  196,  254 

pin,  192 


angles,  108 

flats,  107 
Lag  screws,  210 
Life,  1,  5 

Line  material,  189  to  216 
Loading,  broken  wire,  39  to  42,  193, 
245,  252,  260 

corner,  65,  95,  260 

sleet,  30,  37,  251,  259,  260 

transverse,  65 

wind,  32,  33,  38,  251,  259,  260 
Location,  25 
Loop  cables,  191 


Map  (see  Plan) 


M 


N 


Neighboring  lines,  236 

O 
Outdoor  substation,  142,  143,  145 


Paint,  and  painting,  180,  181,  182, 

255,  263 
Parallelism,  236 
Pin  insulators  (see  Insulators) 
Pins,  metal,  197,  200  to  205,  254 
tests,  200  to  204 
wood,  196  to  199 
Plan,  25,  26,  27 
Poles,  concrete,  152  to  164 

costs,  156,  226 

design,  160  to  164 

erection,  224,  225 

flexibility,  155,  157 

life,  6,  154 

tests,  157,  267 


specifications,  252.  253,  254,  266      Poles,  steel,  curb  line,  113  to  123 


strain,  195 
thro-pin,  194 
Iron  (see  Steel) 


data  on  existing  lines,  140 
design,  120 
latticed,  113  to  123 


INDEX 


271 


Poles,  steel,  triangular,  124 

tests,  118 

Poles,  wood,  data  on  existing  lines, 
lines,  99  to  102 

design,  90  to  94 

life,  6,  99 

settings,  168,  264 

specifications,  84  to  90 

weakest  point  in,  74 
Preservatives,  81  to  84 
Pressure   treatment    (see    Preserva- 
tives) 

Pressure,  wind,  32  to  39 
Profile,  25,  26,  27 

R 

Redwood,  87 

Right  of  way,  3,  12  to  16 

Rivets,  bearing,  105 

shear,  104 

sizes,  106 

spacing,  106 

River  crossings  (see  High  Towers) 
Rock  anchor,  172 


Sag,  comparative,  34 

computation,  47 

curves,  51,  52,  53,  58,  59 

plotting,  27 

tables,  55  to  59 

templet,  27 
Scale,  27 
Shear,  104 
Signs,  250 
Sleet  (see  Loading) 
Spans,  18,  19,  133  to  140 
Specifications,  cement,  265 

concrete,  254,  265 

concrete  poles,  265 

crossings,  248 

galvanizing,  186 

line  construction,  258 

spelter,  185 

steel,  103,  254,  262 

wood  poles,  84  to  90 
Spelter,  185 
Splices,  191 


Steel,  specifications,  103,  246,  254, 

262 

wire,  .46,  57 

Stranding,  246,  253,  261 
Stringing,  217  to  221 
Substations,  142,  143 
Supports  (see  Poles,  Towers),  20 
Survey,  26 

Swinging  contacts,  8,  10 
Switching  station,  143,  145 
Synchronism,  8,  33 


Tables,  angle  sections,  109 

bearing  values,  rivets  and  bolts, 

105 
maximum  sizes,  rivets  and  bolts, 

106 
shearing     values,     rivets     and 

bolts,  104 

strength  of  timber,  73 
Terminal  frames,  141,  145 
Tests  and  testing,  concrete  poles, 

157,  159,  160,  267 
galvanizing,  186 
insulator  pins,  200  to  205 
steel  poles,  118 
Ties,  189,  190,  205 
Timber,  73,  76  to  90,  246 
Towers,  anchors,  171 

bracing,  106  to  110,  125,  127 

costs  (see  Costs) 

data  on  existing  lines,  128,  133 

to  139 

failures,  128,  165 
Transmission     line     crossings     (see 

Crossings) 
Transposition,  141 
Tree  trimming,  8  to  12 

U 

Unit  stress,  70,  72,  256 

V 
Velocity,  35,  36,  37 


272  INDEX 

W  Wire,  tables,  54  to  57 
telephone,  46,  57 

Weatherproof  wire,  55  Wood  (see  Poles) 
Wind  (see  Loading)  decay  and  defects,  77  to  80 

Wire,  aluminum,  45,  56  preservatives,  81 

copper,  43,  54,  55  seasoning,  80,  81 

copper-covered,  44 

properties  of,  50,  59  Y 

steel,  46,  57 

straightening,  218  Yellow  pine,  88 


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